Fresh water runs out faster than you think. 8.3 pounds per gallon — that’s the weight of water, and it’s the first thing offshore mariners learn to respect. A 100-gallon tank sounds like plenty until day 4 of a crossing with four people aboard.

A marine watermaker changes the math entirely. Instead of rationing or cutting voyages short to resupply, you pull fresh water directly from the ocean. The right system is quiet, efficient, and produces more water than your crew can drink. The wrong one fails in the first storm or draws so much power it kills your batteries by midnight.

Here’s what actually matters when you’re choosing one.

Quick Reference: Output Guide by Vessel Type

Vessel / Application	Recommended Output	AMPAC System
Day sailer / weekend cruiser (1–2 people)	30–50 GPD	Compact portable unit
Coastal cruiser / liveaboard (2–4 people)	100–200 GPD	Marine Seawater Desalination Systems
Ocean passage-maker / bluewater yacht (4–8 people)	200–500 GPD	Marine Seawater Desalination Systems
Commercial fishing vessel / charter boat	500–1,500 GPD	Offshore Watermakers
Offshore platform / supply vessel	1,500–10,000+ GPD	Land-Based / Offshore Systems
Emergency / survival situations	30 GPD	Emergency Portable Watermakers

Rule of thumb: plan for 1.5–2 gallons of fresh water per person per day for drinking and basic hygiene. Add 50–100% margin for cooking, washing, and unexpected crew needs.

How a Marine Watermaker Works

A marine watermaker is a reverse osmosis system designed for seawater. The operating principle is the same as any RO system — water is pushed through a semi-permeable membrane under high pressure, leaving dissolved salts, minerals, and contaminants behind.

Seawater has a natural osmotic pressure of roughly 350–400 psi. To push water through the membrane against that gradient, marine RO high-pressure pumps operate at 800–1,000 psi. That pressure forces fresh water molecules through the membrane while 96–99% of dissolved salts stay on the feed side and exit as concentrate (brine) back to the sea.

The typical system has these stages:

Raw seawater intake — through a seacock fitting below the waterline
Strainer — removes debris and marine organisms before they reach filters
Pre-filtration (sediment) — typically 5 micron and 1 micron stages to remove fine particles
High-pressure pump — the core of the system; drives water at 800–1,000 psi against the membrane
RO membrane vessel — one or more spiral-wound TFC (thin-film composite) elements
Product water storage — fresh water output feeds directly to your tanks
Brine discharge — concentrated saltwater exits overboard

Most systems produce 1 gallon of fresh water for every 3–4 gallons of seawater drawn. That’s the “recovery rate” — typically 25–35% for marine seawater systems.

What to Look for When Buying a Marine Watermaker

1. Output Capacity (GPD)

Size for your expected crew at peak consumption, not average. If you’re crossing the Pacific with 6 people, a 100 GPD system will keep you alive. It won’t keep you comfortable. Size up.

Run the watermaker for 2–4 hours per day rather than running a small unit continuously. Frequent short cycles work the high-pressure pump harder; longer cycles are gentler.

2. Power Consumption

This is the spec most buyers underestimate. A 200 GPD system running 2 hours draws roughly 8–12 amp-hours at 12V. On a solar-equipped bluewater cruiser, that’s manageable. On a vessel with limited generating capacity, it’s a real constraint.

Check the specific energy consumption in watts per gallon of product water. Better systems use energy recovery devices on the brine stream to reduce power draw by 20–40%.

3. Component Grade

This is where cheaper systems fall apart. Marine environments are corrosive. Salt air, humidity, vibration, and thermal cycling attack every component.

What to require:
– High-pressure pump: 316L stainless steel or titanium — not regular 316 stainless, not chrome-plated. Stainless with chloride-tolerant alloy.
– Membrane housing: Fiberglass-wound pressure vessels rated for the operating pressure with stainless end caps
– Fittings and valves: 316L stainless or reinforced nylon — never brass below the waterline
– Electrical components: Waterproof to at least IP65; marine-grade wiring with tinned conductors

AMPAC builds all marine watermakers with military and marine-grade certified components — the same specification standard used in offshore and naval applications.

4. Membrane Quality

The membrane is what actually makes the water. TFC (thin-film composite) polyamide membranes are the current standard. For seawater service, look for membranes rated at 800–1,000 psi operating pressure with 99%+ salt rejection.

Avoid systems with membranes specified at reduced salt rejection (some budget systems quote 96–97%). Over time, membrane aging brings that number down. Starting at 97% leaves little margin.

5. Automation and Monitoring

Older systems require manual priming, manual flush cycles, and careful attention to operating pressure. Modern controllers handle this automatically:

Auto-fresh-flush before and after operation (prevents biological growth in the membrane during storage)
Pressure monitoring with auto-shutdown on low feed pressure or membrane over-pressure
Product water TDS monitoring — auto-divert if salinity rises above setpoint
Run-hour tracking for maintenance scheduling

For bluewater passagemakers who run the watermaker unattended overnight, automated TDS divert is important. You don’t want high-salinity water reaching your tanks while you’re asleep.

6. Serviceability Offshore

When something fails 500 miles offshore, you need to fix it. The best marine watermakers use components that are:
– Available worldwide (standard 4040 membrane elements, standard pump seals)
– Accessible without specialized tools
– Replaceable by a reasonably competent sailor with basic mechanical skills

Ask what consumables the system requires and whether they’re available from distributors in the regions where you cruise.

Sizing Calculation: How Much Water Do You Actually Need?

Person-days × consumption	Daily requirement
1 person, minimal (drinking only)	~0.5 GPD
1 person, conservative (drinking + cooking)	~1 GPD
1 person, comfortable (drinking, cooking, hygiene)	1.5–2 GPD
1 person, generous (includes light washdown)	2.5–3 GPD

Example: 4 people on a 10-day passage, comfortable use = 4 × 2 = 8 GPD needed.

If you run the watermaker 2 hours per day, you need a system producing at least 4 GPH (96 GPD). A 100–150 GPD system with daily 1.5–2 hour runtime covers this with margin.

Add 20–25% extra for cooking, washing dishes, and unexpected water needs. The math points to 120–200 GPD for a 4-person bluewater vessel doing extended passages.

Maintenance: Keeping a Marine Watermaker Running for 10+ Years

The two things that kill marine watermakers are:

Membrane biofouling — biological growth inside the membrane during storage
Corrosion — seawater attacking metal components between use

Prevent biofouling: flush and preserve properly

After each use, flush the membrane with fresh water for at least 5–10 minutes. This displaces the concentrated seawater inside the membrane elements, preventing biological growth.

For storage longer than 5–7 days, add a membrane preservative (sodium metabisulfite solution) to the system and keep it wet. Dry storage causes irreversible membrane damage within weeks.

Monthly checks (if using regularly)

Product water TDS: should be < 500 mg/L for seawater feed
High-pressure pump: check for vibration, unusual noise, or seal leaks
Pre-filters: inspect for color and flow restriction (replace when discolored or flow drops)
All connections: check for seawater weeping at fittings

Annual service

Replace pre-filters regardless of visual condition
Chemical clean the membrane (if TDS is trending up or flux has declined by > 15%)
Inspect high-pressure pump seals and replace if weeping
Check and lubricate all valves

AMPAC provides replacement filters, membranes, and accessories for marine systems, along with technical support for field service.

AMPAC Marine Watermaker Range

AMPAC USA has built marine and offshore watermakers since 1990. All systems are manufactured in the USA with military and marine-grade certified components.

Marine Seawater Desalination Systems — From compact 30 GPD yacht units to 1,500 GPD commercial systems. Automated control, modular design, and components rated for offshore conditions.

Offshore Watermakers — For commercial vessels, supply ships, and platforms. High-output systems designed for continuous operation in harsh marine environments.

Emergency Portable Watermakers — Hand-operated and powered portable units for life raft and survival situations. SOLAS-compliant options available.

Land-Based Seawater Systems — For island resorts, coastal facilities, and remote installations drawing from seawater or brackish sources.

Talk to our marine systems engineers →

Frequently Asked Questions

Q: What is a marine watermaker?
A: A marine watermaker is a reverse osmosis desalination system installed on a boat, yacht, or vessel that converts seawater into fresh drinking water. It uses a high-pressure pump to force seawater through a semi-permeable membrane, removing 96–99% of dissolved salts.

Q: How much does a marine watermaker cost?
A: Entry-level compact systems for small vessels start around $1,500–$3,000. Mid-range bluewater cruiser systems (100–300 GPD) typically cost $4,000–$10,000. Commercial and offshore systems range from $15,000 to $100,000+ depending on capacity. Total cost includes installation materials and initial consumables.

Q: How long does a marine watermaker membrane last?
A: With proper flushing and storage procedures, a marine RO membrane typically lasts 3–7 years. The main failure modes are biofouling from improper storage and scaling from high-TDS or hard water. Membranes exposed to chlorinated fresh water during flushing may degrade faster.

Q: Can I use a marine watermaker in turbid or coastal water?
A: Yes, with appropriate pre-filtration. Turbid coastal water requires finer and more frequent pre-filter changes. Operating in harbor with high turbidity or algae bloom conditions can foul pre-filters quickly — plan for more frequent filter changes if regularly departing from coastal areas with poor water quality.

Q: What’s the difference between a marine watermaker and a regular RO system?
A: Marine watermakers operate at much higher pressures (800–1,000 psi vs. 150–300 psi for brackish/tap RO), use membranes specifically designed for seawater salt concentrations, and are built with corrosion-resistant marine-grade materials throughout. A residential or commercial RO system will not function properly with seawater feed.

Q: How do I store a marine watermaker when not in use?
A: For short-term storage (< 1 week), flush with fresh water and keep wet. For medium-term (1 week – 3 months), add sodium metabisulfite preservative solution and keep the membrane wet. For long-term storage (> 3 months), drain, preserve with a more concentrated biocide solution, and store the membrane element in a sealed, moist environment.

References

U.S. Coast Guard, “Marine Watermaker Safety and Installation Guidelines,” COMDTINST M16672.
International Association of Classification Societies (IACS), freshwater generator standards.
Elimelech, M., Phillip, W.A. (2011). “The Future of Seawater Desalination: Energy, Technology, and the Environment.” Science, 333(6043), 712–717.
NSW Government, “Seawater Desalination for Marine Applications,” Water Efficiency Guide.
AMPAC USA, Marine Seawater Desalination Watermakers product documentation.

Published May 2026. Consolidates: “Watermakers Make Your Life Easy at the Sea,” “An All-You-Need-To-Know Guide on Marine Watermakers,” “What Makes Marine Watermaker from AMPAC USA Reliable,” “Recognizing the Need for Marine Watermakers,” “Select Marine Water Maker,” and “6 Pointers to Remember Before Buying a Marine Watermaker.”

Marine Watermaker Buyer’s Guide: How to Choose, Size, and Maintain a Seawater Desalination System