My wife and I make marine water heaters/calorifiers as a means to enjoy out Bavaria 38 Ocean. Our company is called Torrid Marine and is based in Washington State. Though our market is generally larger boats, we have experience with all sizes of boats and marine fresh water systems
Wear and tear, including internal corrosion is a result of a few factors including: Amount of use/non-use. Flow through the water hearer (including bigger boats that rely on constant circulation), source water pH levels, water treatment and regular inspection and maintenance. As a rule we use at least one inch of magnesium anode per gallon of calorifier capacity. Their use is not an option in my opinion.
Our marine water heaters are designed to last a long life, if properly inspected and maintained. We believe it is easier and much less expensive to take care of it than to have to replace it every couple years. We regularly have folks call us to ask if something is wrong as their water heater is still running trouble free after 30 plus years. Mileage will vary.
Knowledge and preventative maintenance, plus having the right spare parts when you need them are key to a long lasting, reliable arine water heater. We recommend that the water heater be drained in the boat is going to be laid up for the off season. If the boat is in use year-round, we recommend that the water heater be shut off from electrical source and drained after a day of cool-down. If it is newer (within 2 years from installation) when convenient, the unit should be emptied and the anode(s) removed and inspected for depletion. If more than 50 percent of the magnesium is missing or the extrusion rod is visible anywhere on the anodes, they should be replaced. The best case scenario is this happens annually. Generally it does not.
If you are unaware of the last time an inspection/replacement has occurred, assume it has not been done in the past. We recommend the same shut-down, cool-down drain followed by a visual interior inspection using the electrical element port as access into the inner pressure vessel. This port is preferable as it has a simple o-ring seal and can be refitted without the use of sealant. If there is extensive/unknown corrosion the risk of catastrophic damage is much lower using this as a entry/viewing portal (i.e. all the rest risk that you might unknowingly break a corrosion seal and cause a major, generally irreparable leak). Inspection is easiest with a lighted flexible endoscopic-like probe with which you can have a good look around on the inside.
It is important to note that desalination or reverse osmosis by its very nature produces very reactive water. Fresh water will seek out remineralization if it has been produced onboard as the pH will generally be below neutral or slightly acidic. Some boats add calcium carbonite fitration as a rebuffer to reduce the corrosive properties of RO Water.
Finally, cool water is less reactive than hot water. If the boat isn't being used, there is little sense to heat the water unless there is a risk of freezing.
Hope this helps.
