Fresh water is the resource that decides how long you can stay out. Fuel you can plan around, food keeps, batteries you can recharge with the sun. Water, on most cruising boats, is the one that sends you back to the dock. A watermaker changes that equation, but it also adds a power-hungry, maintenance-hungry system to a boat that already has plenty of both. Before you spend three to eight thousand euros on one, it's worth being honest about how you actually cruise, what your energy budget looks like, and what a reverse-osmosis membrane really wants from you.

What a watermaker actually does

Every marine watermaker on the market works the same way : a high-pressure pump forces seawater through a semi-permeable membrane at roughly 55 to 60 bar. The membrane rejects salt, most bacteria and most dissolved solids. What comes out the product side is drinkable fresh water, typically around 200 to 500 ppm TDS. What goes overboard is a concentrated brine stream, usually four to six times the volume of the product water.

Output is measured in litres per hour. Small cruising units make 30 to 60 L/h. Mid-range units sit at 60 to 150 L/h. Anything above that is charter-boat or superyacht territory. The number to remember is not the peak output, it's litres per hour per amp, because that ratio decides whether you can run the thing off your house bank or whether you need to fire up the genset every day.

Are you actually the right customer for one

A watermaker earns its place on some boats and is dead weight on others. The honest test is how many nights per year you spend away from a dock hose. If you weekend out of a marina and take two weeks in August, you almost certainly don't need one. Two 200-litre tanks and a bit of discipline will cover it. If you're crossing an ocean, cruising the Cyclades for a month, or living aboard, the calculation flips completely.

Rough guide, and it is only a guide :

  • Under 30 nights a year off-dock : skip it. Buy jerry cans, fix your tank gauge, and put the money into batteries and solar.
  • 30 to 90 nights, mostly coastal : marginal. A small 30 to 60 L/h unit can work if you have the power to run it.
  • Over 90 nights, or any bluewater plans : yes, and size it properly the first time.
  • Charter operation : almost always yes, because guest water consumption is roughly double what owners use, and running out mid-charter is a refund conversation.

Owner consumption on a well-run cruising boat sits around 10 to 20 litres per person per day. Guests on charter typically burn 40 to 80. Do that math against your tank capacity before you shop.

Power is the real constraint, not the plumbing

This is where most watermaker projects go wrong. The membrane and the high-pressure pump are the easy part. Feeding them electrically is what breaks the budget.

A typical 60 L/h 12 V DC unit draws around 20 to 25 amps while running. To make 120 litres, you're pulling roughly 40 to 50 Ah out of the bank, plus whatever the boost pump and controller add. On a 400 Ah lithium bank with 600 W of solar, that's manageable in summer. On a tired lead-acid bank in October, it isn't.

Three power architectures dominate :

  1. 12 or 24 V DC units. Simple, run directly off the house bank, best for sailboats with strong solar. Output is limited, usually 30 to 90 L/h.
  2. AC units run off an inverter or genset. Higher output, but you're either running the genset (noisy, thirsty) or double-converting through an inverter (lossy).
  3. Energy-recovery units. Use a pressure-exchange device to recycle the brine's pressure. Roughly halves the amps per litre. More expensive, more moving parts, but on a solar-heavy boat they're a genuine game-changer.

Before you spec anything, do a proper energy audit. If you haven't already sized your bank around your existing loads, that's the first job, not the watermaker. The same logic applies to heating and any other continuous-draw system you add to a cruising boat : the electrical budget decides what's realistic long before the marine catalogue does.

Modular or compact, and where it fits

Watermakers come in two physical formats. Compact units put the pump, membrane and controls on a single frame. Easy to install, easy to service, but they need a rectangular hole in a locker roughly the size of a small suitcase. Modular units split the components so you can tuck the pump under a bunk, the membrane along a hull stringer and the control panel at the nav station. More flexible, more plumbing, more places for a leak to hide.

Wherever it goes, the install needs :

  • A dedicated through-hull for the raw-water intake, ideally forward of the engine intake and never sharing a seacock with anything else.
  • A brine discharge above the waterline where you can see it. You want to know instantly if flow stops.
  • A pre-filter stage, usually 20-micron then 5-micron, in a place you can actually reach without disassembling the galley.
  • A freshwater flush plumbed in from day one. Not optional. This single feature does more to extend membrane life than anything else.
  • A diverter valve so the first minute of product water goes overboard, not into your tank.

Access matters more than tidiness. You will change pre-filters every 30 to 100 running hours. If that job requires removing a floorboard and a fridge compressor, you'll stop doing it, and the high-pressure pump will pay the price.

Membranes, fouling and the jobs nobody mentions in the brochure

The membrane is a consumable. Treated well, it lasts five to seven years. Treated badly, it lasts one season. "Treated well" means three things :

  • Run it regularly. A membrane that sits wet and stagnant for two weeks grows biofilm. Either run the unit at least twice a week, or freshwater-flush it after every use and pickle it (glycerine and sodium metabisulfite solution) for any layup longer than two to three weeks.
  • Keep it away from hydrocarbons. Never run the watermaker in a marina, near a fuel dock, or when there's a visible sheen. Diesel destroys the polyamide membrane permanently and instantly.
  • Watch the pre-filters. A clogged pre-filter starves the high-pressure pump and shortens its life. A pre-filter you never change lets fine particles reach the membrane.

Budget roughly 150 to 400 euros a year in filters, flush cartridges, pickling chemicals and the occasional membrane clean. A replacement membrane is 300 to 800 euros. A high-pressure pump rebuild kit is similar. None of this is optional, and none of it appears in the sales quote.

Monitoring what the system is telling you

Every watermaker has three numbers that matter : feed pressure, product flow rate and product water quality (TDS or conductivity). If pressure climbs while flow drops, your membrane is fouling. If quality degrades while pressure holds, the membrane seal or the membrane itself is failing. If pressure won't build at all, look at the pre-filter or the high-pressure pump.

Better units log these values. On a modern boat with a NMEA 2000 network, you can bring tank levels, watermaker status and battery draw onto the same screen, and this is where trend data becomes useful. A single-point reading tells you nothing. A three-month trend of feed pressure at a given water temperature tells you exactly when to plan the next service. If you already have telemetry logging engine hours, fuel and tank levels for maintenance planning, adding watermaker runtime and consumption into the same picture is straightforward and pays for itself the first time you catch a problem before it becomes a repair.

The same monitoring habit applies to the boat's other continuous systems : cooling, charging, fridge, autopilot. Once you're used to seeing what "normal" looks like, "abnormal" jumps out weeks before it becomes a failure. This is also why owners moving toward long-distance cruising tend to think about reliable connectivity at the same time as watermakers : the two together turn a boat from a weekend platform into something you can genuinely live on.

The honest verdict

A watermaker is worth it when your cruising style pushes you past what your tanks can carry, when your electrical system can feed it without ruining the rest of your day, and when you're willing to treat it as a regular maintenance item rather than a fit-and-forget appliance. For the right boat and the right owner, it's genuinely liberating : you stop planning routes around fresh-water resupply and start planning them around weather and where you want to be. For the wrong boat, it's an expensive locker occupant that fails at the worst possible moment because it hasn't been run in six weeks.

The real question isn't whether watermakers are worth it in general. It's whether your boat, with your power budget and your cruising calendar, can carry one honestly. If you can't answer that with real numbers from a season of logged data, that's probably the place to start before the shopping does.