{"id":88925,"date":"2026-05-13T21:57:06","date_gmt":"2026-05-13T21:57:06","guid":{"rendered":"https:\/\/www.ampac1.com\/blog\/marine-watermaker-buyers-guide\/"},"modified":"2026-05-13T21:57:06","modified_gmt":"2026-05-13T21:57:06","slug":"marine-watermaker-buyers-guide","status":"publish","type":"post","link":"https:\/\/www.ampac1.com\/blog\/marine-watermaker-buyers-guide\/","title":{"rendered":"Marine Watermaker Buyer’s Guide: How to Choose, Size, and Maintain a Seawater Desalination System"},"content":{"rendered":"

Fresh water runs out faster than you think. 8.3 pounds per gallon \u2014 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.<\/p>\n

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.<\/p>\n

Here’s what actually matters when you’re choosing one.<\/p>\n

\n

Quick Reference: Output Guide by Vessel Type<\/h2>\n\n\n\n\n\n\n\n\n\n\n
Vessel \/ Application<\/th>\nRecommended Output<\/th>\nAMPAC System<\/th>\n<\/tr>\n<\/thead>\n
Day sailer \/ weekend cruiser (1\u20132 people)<\/td>\n30\u201350 GPD<\/td>\nCompact portable unit<\/td>\n<\/tr>\n
Coastal cruiser \/ liveaboard (2\u20134 people)<\/td>\n100\u2013200 GPD<\/td>\nMarine Seawater Desalination Systems<\/a><\/td>\n<\/tr>\n
Ocean passage-maker \/ bluewater yacht (4\u20138 people)<\/td>\n200\u2013500 GPD<\/td>\nMarine Seawater Desalination Systems<\/a><\/td>\n<\/tr>\n
Commercial fishing vessel \/ charter boat<\/td>\n500\u20131,500 GPD<\/td>\nOffshore Watermakers<\/a><\/td>\n<\/tr>\n
Offshore platform \/ supply vessel<\/td>\n1,500\u201310,000+ GPD<\/td>\nLand-Based \/ Offshore Systems<\/a><\/td>\n<\/tr>\n
Emergency \/ survival situations<\/td>\n30 GPD<\/td>\nEmergency Portable Watermakers<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Rule of thumb: plan for 1.5\u20132 gallons of fresh water per person per day for drinking and basic hygiene. Add 50\u2013100% margin for cooking, washing, and unexpected crew needs.<\/em><\/p>\n

\n

How a Marine Watermaker Works<\/h2>\n

A marine watermaker is a reverse osmosis system designed for seawater. The operating principle is the same as any RO system \u2014 water is pushed through a semi-permeable membrane under high pressure, leaving dissolved salts, minerals, and contaminants behind.<\/p>\n

Seawater has a natural osmotic pressure of roughly 350\u2013400 psi. To push water through the membrane against that gradient, marine RO high-pressure pumps operate at 800\u20131,000 psi. That pressure forces fresh water molecules through the membrane while 96\u201399% of dissolved salts stay on the feed side and exit as concentrate (brine) back to the sea.<\/p>\n

The typical system has these stages:<\/p>\n

    \n
  1. Raw seawater intake<\/strong> \u2014 through a seacock fitting below the waterline<\/li>\n
  2. Strainer<\/strong> \u2014 removes debris and marine organisms before they reach filters<\/li>\n
  3. Pre-filtration (sediment)<\/strong> \u2014 typically 5 micron and 1 micron stages to remove fine particles<\/li>\n
  4. High-pressure pump<\/strong> \u2014 the core of the system; drives water at 800\u20131,000 psi against the membrane<\/li>\n
  5. RO membrane vessel<\/strong> \u2014 one or more spiral-wound TFC (thin-film composite) elements<\/li>\n
  6. Product water storage<\/strong> \u2014 fresh water output feeds directly to your tanks<\/li>\n
  7. Brine discharge<\/strong> \u2014 concentrated saltwater exits overboard<\/li>\n<\/ol>\n

    Most systems produce 1 gallon of fresh water for every 3\u20134 gallons of seawater drawn. That’s the “recovery rate” \u2014 typically 25\u201335% for marine seawater systems.<\/p>\n

    \n

    What to Look for When Buying a Marine Watermaker<\/h2>\n

    1. Output Capacity (GPD)<\/h3>\n

    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.<\/p>\n

    Run the watermaker for 2\u20134 hours per day rather than running a small unit continuously. Frequent short cycles work the high-pressure pump harder; longer cycles are gentler.<\/p>\n

    2. Power Consumption<\/h3>\n

    This is the spec most buyers underestimate. A 200 GPD system running 2 hours draws roughly 8\u201312 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.<\/p>\n

    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\u201340%.<\/p>\n

    3. Component Grade<\/h3>\n

    This is where cheaper systems fall apart. Marine environments are corrosive. Salt air, humidity, vibration, and thermal cycling attack every component.<\/p>\n

    What to require:
    \n– High-pressure pump:<\/strong> 316L stainless steel or titanium \u2014 not regular 316 stainless, not chrome-plated. Stainless with chloride-tolerant alloy.
    \n– Membrane housing:<\/strong> Fiberglass-wound pressure vessels rated for the operating pressure with stainless end caps
    \n– Fittings and valves:<\/strong> 316L stainless or reinforced nylon \u2014 never brass below the waterline
    \n– Electrical components:<\/strong> Waterproof to at least IP65; marine-grade wiring with tinned conductors<\/p>\n

    AMPAC builds all marine watermakers with military and marine-grade certified components \u2014 the same specification standard used in offshore and naval applications.<\/p>\n

    4. Membrane Quality<\/h3>\n

    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\u20131,000 psi operating pressure with 99%+ salt rejection.<\/p>\n

    Avoid systems with membranes specified at reduced salt rejection (some budget systems quote 96\u201397%). Over time, membrane aging brings that number down. Starting at 97% leaves little margin.<\/p>\n

    5. Automation and Monitoring<\/h3>\n

    Older systems require manual priming, manual flush cycles, and careful attention to operating pressure. Modern controllers handle this automatically:<\/p>\n