One thing to note - hot water on a boat is only useful if you have a practical shower. My boat, for example, does not (yes, head is large and there is a grate on the floor, and at one time the faucet had a shower head - but head walls, and lazarette doors are all mahogany and teak, including the main bulkhead and compression post - I don't think they would appreciate splashing water). So, my boat has no water heater and honestly I never miss it - less stuff to worry about. On all of my previous boats situation was not much different - there was a heater, but heads were very rarely of sufficient size or convenience to bother with a shower (unless you like showering while sitting on the toilet bowl
).