sulfa smell in water
The anode can cause that smell. Flushing and using bleach usually doesn't help. You have to flush the system by adding hydrogen peroxide. A pint in 20 gallons of water. It is better to actually pour the peroxide in the hot water tank. That might be tough to do in a boat.
Then you have to replace the anode.
There is a lot of info on the Internet concerning this subject