In order for us to be able to provide more relevant advice, it would be helpful to fill out profile information to identify the type of boat you have.
It would also be helpful to identify the water
heater you presently have, whether you find it adequate, and whether you want to replace it with an identical type.
I presently have an Ariston 4 gallon electric
water heater. It is very easy to replace and (as Dockhead states) requires no more effort/skill than a basic household plumbing project
. One thing I am considering doing, which it something you could keep in mind if putting in a new unit. The marinas
in my area tend to have very hard water and I am aware of boats (over the years) having issues with calcification blocking water flow into and out of the water heater. I am planning on plumbing a flush out system to I can periodically purge it.