The anode protects you from corrosion. In our home water which is heavy on iron sulfide supposedly you can replace the conventional anode with something else or get rid of the anode (shortening the life of your water heater) and it is supposed to get rid of the smell. We also have a problem only with the hot water. In our case, the odor is a harmless but smelly bacteria and we can eliminate it by keeping the temperature pretty hot. That wouldn't work in your situation as you probably only heat up the water when you are on the boat. Flushing the tank and filling it from a different water source might solve your problem. If not, SG's solution would be your next step.