Fresh water is weight. At 8.3 lbs per gallon, a 200-gallon water tank adds 1,660 lbs to your boat. For a coastal cruiser making weekend trips, tank capacity is rarely an issue. But for boats doing extended passages, island hopping in the Bahamas, or chartering in remote areas, water supply becomes a logistical constraint that affects how long you can stay out and how many showers your crew gets.
A watermaker (reverse osmosis desalination system) converts seawater into fresh water onboard, giving you independence from dock water and jerry jugs. Modern marine watermakers are reliable, energy-efficient, and — when properly maintained — produce water that's cleaner than most municipal supplies.
The process is straightforward in concept: seawater is pressurized to 800-1,000 PSI and forced through a semi-permeable membrane. The membrane allows water molecules to pass through while rejecting 98-99.5% of dissolved salts, bacteria, and other contaminants. The filtered fresh water (permeate) goes to your tank. The concentrated brine (reject water) is discharged overboard.
Water consumption on boats varies widely based on crew size, lifestyle, and equipment. General guidelines:
San Rafael, California-based Spectra specializes in energy-efficient, DC-powered systems for cruising sailboats and smaller power boats. Their Clark Pump technology uses an energy recovery device that dramatically reduces power consumption — the Spectra Ventura 150T produces 6 gallons/hour while drawing only 4 amps at 12V. That's compatible with solar panel charging and makes watermaking practical on sailboats without generators.
Spectra units are compact, relatively quiet, and well-supported. Prices range from $6,000 (Ventura 150) to $14,000 (Catalina 340) for the unit itself, plus $1,000-$3,000 for installation.
Sea Recovery is the dominant brand on yachts over 50 feet. Their Aqua Whisper line (for mid-range boats) and Ultra Whisper/HRO lines (for large yachts) are found on thousands of production and custom yachts worldwide. Sea Recovery systems are AC-powered, well-proven, and supported by a global dealer network. Prices range from $8,000 (Aqua Whisper DX) to $35,000+ (HRO-series for large yachts) before installation.
Village Marine Tec builds commercial-grade watermakers for yachts, military vessels, and offshore platforms. Their Little Wonder Marine (LWM) line serves the 40-80 foot market, while the STW line handles large yachts and commercial vessels. Known for robust construction and long service life. Priced similarly to Sea Recovery.
A watermaker installation involves plumbing, electrical, and potentially structural work. Key decisions include:
Professional installation typically costs $2,000-$5,000 on top of the unit price, depending on complexity. A straightforward install on a boat with existing through-hulls and accessible mounting space runs less than a retrofit into a tight engine room on an older boat.
Watermaker maintenance is simple but absolutely critical. A neglected watermaker is an expensive paperweight.
If your output water TDS is climbing above normal baseline, the membrane is either fouled, damaged, or aging out. Try a membrane cleaning cycle first. If TDS doesn't improve, the membrane may need replacement. Also check that the brine seal on the membrane element isn't bypassed — if seawater can flow around the membrane instead of through it, TDS will be high.
Reduced water production usually indicates clogged pre-filters (check and replace), a worn high-pressure pump (check pressure output), or a fouled membrane (clean per schedule). If the boat is in a harbor with high sediment or algae levels, pre-filters will clog faster.
Air in the intake line (check through-hull, strainer, and hose connections for leaks above waterline), a failed check valve, worn pump valves, or a ruptured high-pressure hose can all prevent the system from reaching operating pressure. Start at the intake and work toward the pump.
The most common cause of premature membrane failure. If the system sits idle without flushing for more than a week, biological growth colonizes the membrane surface. Prevention is simple: flush on schedule. Treatment requires chemical cleaning. Severe biofouling may require membrane replacement ($300-$800).
If the boat will be inactive for more than two weeks, pickle the membrane with a preservative solution (sodium metabisulfite is most common, $15-$25 per treatment). This prevents biological growth during the dormant period. When recommissioning, flush the preservative thoroughly and divert the first 10-15 minutes of product water overboard before sending it to your tank.
For boats that are hauled for the season, your haul-out prep checklist should include watermaker pickling as a standard step.
Technically yes, but the water quality in many marinas is poor — fuel residue, antifouling paint chemicals, sewage discharge, and high biological loads. Running the watermaker in a dirty harbor can foul the membrane prematurely and produce water with off-tastes. Most cruisers make water offshore or in clean anchorages and top off at the dock with hose water when needed.
The primary cost is energy. A Spectra Ventura 150 (DC powered, 6 gal/hr) draws about 4 amps at 12V, or roughly 48 watts. At $0.40/kWh (generator fuel cost), that's about $0.003 per gallon. An AC-powered Sea Recovery Aqua Whisper making 25 gal/hr at 1.5 kW costs about $0.024 per gallon. Add membrane replacement ($300-$800 every 3-7 years), filters ($100-$200/year), and pump maintenance ($200-$500 every few years), and the all-in cost is roughly $0.05-$0.15 per gallon. Marina water costs $0.01-$0.05 per gallon where it's metered.
It depends on your cruising style. A 45-foot boat with 200 gallons of tank capacity and a crew of two can go 5-7 days at comfortable usage rates without refilling. If you're coastal cruising and hitting marinas regularly, a watermaker may be unnecessary. If you're crossing oceans, spending weeks in remote anchorages, or chartering with guests who expect unlimited water, a watermaker changes how you use your boat.
Multiply your crew count by daily water consumption (20-40 gal/person for cruising comfort), then divide by the number of hours you want to run the watermaker per day. For a couple on a 40-foot sailboat wanting 50 gallons/day with 3-4 hours of runtime: you need roughly 13-17 gallons/hour capacity. A Spectra Ventura 200 (8.5 gal/hr) or Catalina 340 (14 gal/hr) would fit, as would a Sea Recovery Aqua Whisper DX (25 gal/hr, which gives you room to grow). Always size slightly larger than your calculated need — membrane output decreases as water temperature drops and as the membrane ages.