Read before you buy.

Brochures give you litres per hour and a wattage. Divide one by the other and the whole market rearranges itself.

Same pump, same power draw, nearly double the water. The membrane pack is doing all the work.

Two different mechanisms, the same idea: stop throwing the brine's pressure overboard.

The unit is the part everyone quotes. Installation, consumables and the energy to run it decide what you actually pay over ten years.

Size from your crew's real daily water use, not from boat length. Then check the answer against the power you can actually spare.

Not a sponsored list. Every system we can source figures for, ranked on the one spec that decides whether it suits your boat.

A portable unit avoids the through-hulls and the invoice that comes with them. It also has to be set up, stowed and lived with every time you use it.

The voltage decides where the energy comes from, how thick the cable has to be, and whether you can make water without starting anything.

The part of the price nobody quotes up front. Here is what the job actually consists of, and the questions that make two quotes comparable.

Much of the work is ordinary boat plumbing and wiring. Two parts of it are not, and they are the parts that sink boats.

For some boats it is the best money you will spend; for others it is five figures of complication. The honest answer turns on three things.

Three ways to solve the same problem, with very different costs. Work out which one your boat actually has before you spend.

A second-hand system can be a bargain or an expensive membrane holder. The difference is almost always how it was laid up.

Watermakers reward regular use and correct shutdown. Most failures start with seawater left sitting inside a membrane.

Low output, no pressure, salty product water: each points somewhere different. Work through the cheap causes first.

Membrane area is what turns pump power into litres. It is also the spec that decides whether a system is efficient or thirsty.

Making water on sunshine alone is entirely achievable — but only with the right system. The arithmetic is unforgiving and completely predictable.

Wh/L, Clark pump, ERS, TDS, brine, pickling. Plain definitions for the words dealers use without explaining them.