Marine Watermakers · How it works

How Does a Marine Watermaker Work? Reverse Osmosis, Explained Plainly.

A marine watermaker turns seawater into fresh, drinkable water by pushing it through a very fine membrane at high pressure. Seawater in, fresh water out.

Short versionA marine watermaker turns seawater into fresh, drinkable water by pushing it through a very fine membrane at high pressure. A portion of the water passes through the membrane as freshwater, while the remaining seawater carries the rejected salt back overboard. Seawater in, fresh water out.

That's the whole idea in a sentence. Turning seawater into drinking water like this is called desalination, or marine desalination when it happens on a boat. But if you're about to trust one to keep your tanks full a hundred miles from the nearest marina, it helps to know what's actually going on inside.

01Start with osmosis, then flip it

Osmosis happens naturally. If fresh water and salty water are separated by a semi-permeable membrane, water moves through the membrane toward the saltier side, tending to reduce the difference in concentration.

Reverse osmosis makes water travel in the opposite direction by applying pressure to the seawater side of the membrane. That pressure forces a portion of the water through the membrane, while most of the dissolved salt remains in the seawater flowing across it. The water that passes through becomes fresh product water. The remaining seawater becomes more concentrated and carries the rejected salts away as brine.

Osmosis vs reverse osmosis

Natural osmosis

Left alone, water drifts across the membrane toward the saltier side.

Reverse osmosis ≈ 800 psi

Add pressure and the flow reverses. Fresh water crosses the membrane, the dissolved salt is carried away as brine.

To reverse the natural flow of osmosis, the system must apply pressure greater than the osmotic pressure of seawater. Conventional marine watermakers commonly operate at around 800 psi to create enough additional driving pressure to produce freshwater at a useful rate. Producing that pressure is the job of the high-pressure pump, the heart of the whole system.

02The journey of a drop, step by step

Follow a single drop of seawater through a watermaker and the process makes sense quickly. Scroll down the path from intake to tank.

The journey of a drop · intake to tank

Seawater comes aboard

It's drawn in through a fitting below the waterline.

The strainer catches the big stuff

A sea strainer pulls out weed, sand and anything else you don't want in the system.

Pre-filters clean it up

A low-pressure feed pump supplies seawater to the system and pushes it through staged pre-filters that remove finer sediment, commonly down to around five microns. This helps protect the high-pressure pump and membrane, maintain stable feed flow and extend the service life of the system.

The high-pressure pump does the heavy lifting

The filtered seawater is raised to the pressure required for reverse osmosis, typically around 800 psi under normal seawater conditions, and then sent through the membrane vessel.

The membrane splits the stream

Under pressure, a portion of fresh water passes through the membrane, while dissolved salts and contaminants remain in the seawater flowing across it. A quality seawater reverse-osmosis membrane typically rejects more than 99% of dissolved salts when operated under the correct conditions.

Two streams come out

One stream is fresh product water, which is tested before being directed into the tank. The other is brine, the remaining seawater carrying the rejected salt, which is discharged back overboard. Only a portion of the incoming seawater becomes freshwater. The remaining flow carries rejected salts away from the membrane.

A quick quality check

When the watermaker first starts, the product water is normally diverted to a test outlet. Once its salinity is within the acceptable range and the system is operating correctly, it can be directed into the freshwater tank.

Seawater in one end, fresh water out the other. That's all there is to making fresh water from seawater: no chemicals, no boiling, just pressure and a specialised reverse osmosis membrane.

03What runs it?

Watermakers are built to suit how you power your boat. Marine watermakers can run from 12V or 24V DC power, or from 230/240V AC supplied by an inverter, generator or shore power. The right setup depends on how much freshwater you need, how quickly you want to refill the tanks, and the battery, charging, inverter or generator capacity already available onboard.

04Looking after it

Reverse osmosis watermakers are refreshingly low-drama if you keep up with the basics. Rinse the pre-filters, change them when they're dirty, and flush the system with fresh water when you won't be using it for a while. Freshwater flushing removes stagnant seawater from the system and helps limit biological growth during periods between operation. If the watermaker will be unused for an extended period, the membrane may need to be preserved (pickled) with an approved storage solution and must never be allowed to dry out.

One thing worth checking before you buy is whether a system uses standard, off-the-shelf parts. Systems built around standard, non-proprietary membranes, filters and service components are generally easier and more economical to maintain, without locking the owner into one manufacturer for every replacement part.

05Why it's worth it

Fresh water is usually the thing that decides how long you can stay out. Run out and you're heading back, no matter how good the fishing is. A watermaker takes that limit away. You make water as you go, so the tanks are never the reason the trip ends early. That's the real payoff: less planning around jerry cans, and more time on the water.

06Marine watermaker FAQs

Yes, reverse-osmosis product water can be suitable for drinking provided the system is correctly installed, operated and maintained, and the freshwater tank and downstream plumbing are kept clean and hygienic. Product-water salinity should always be checked before the water is directed into the tank. A TDS reading indicates the level of dissolved solids, but it does not by itself confirm microbiological safety or tank cleanliness.

Water Makers Australia systems are available in configurations producing up to 240 litres per hour for families, liveaboards and boats with higher freshwater needs.

Yes, provided the surrounding seawater is reasonably clean, the intake remains fully submerged and the system is receiving a reliable seawater supply. Avoid operating the watermaker near fuel or oil spills, sewage outlets, muddy river mouths or areas affected by heavy sediment and contaminated runoff.

Yes, provided the seawater feed remains uninterrupted and free of air.

Mostly it's just pre-filter changes and the occasional fresh-water flush. A reverse-osmosis membrane can last for years when it receives clean feed water, adequate concentrate flow and correct flushing, operation and preservation.

Power consumption varies between systems, so the most useful comparison is how much energy the watermaker uses to produce each litre of freshwater. Match the system to your power setup and it's an easy fit.

Fresh Water, Endless Horizons

Fresh water, wherever you drop anchor.

A marine watermaker works on a simple principle: seawater is filtered, pressurised and separated by a reverse-osmosis membrane to produce freshwater onboard. If you'd like a hand matching a system to your boat and the way you cruise, the team at Water Makers Australia can talk it through with you, backed by a 3-year warranty and 7-day Aussie support.

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