Believe it or not, a watermaker can kill a marine engine. Surprise: De-Salinated water is very corrosive. Watermakers with their reverse-osmosis membranes, scrub saltwater squeaky clean until it's fit for drinking. But when RO water is added to an engine's expansion tank to top off the coolant, the reverse-osmosis-generated water attacks copper and gray iron castings. Components like temperature senders literally dissolve in the mineral-starved water. Overheating and premature engine death can result. Don't do it.
One case history documented by Seattle-based Northern Lights/Lugger reveals how reverse-osmosis-generated water dissolved about a dozen temperature senders in a row before the warranty team finaly figured out what the problem was. Here's something scary. There have been cases where cylinder liners eroded completely through in as few as 300 engine hours. But don't despair, there's a simple solution.
According to, Dick Gee, of Northern Lights, "The only fluid that belongs in an engine's coolant blend, besides antifreeze, is de-ionized distilled water."