Russ, where will you be cruising? If boat parts
are easily available I'd not replace a working water heater. If you will be cruising where a replacement heater would be hard to get I would consider replacing it now. On the other hand, lots of things built 15 years ago were better quality and longer lasting than new things today. Have you inspected the heater carefully? Is the tank clean and free of rust and scale? Perhaps taking spare parts
for the heating
element, wires, hoses and clamps would be better than replacing the whole unit.