It's easy!
I replaced the water heater in my 1980 Hunter 30 some years ago. Mine was located in the starboard cockpit lazarette. Look in the West Marine or Boat/US catalogs for any of the 6 gallon models. Any should fit, but measure to make sure. The installation is easy and you may leave off the hose connections to the engine if you prefer. My engine (Yanmar 2QM15) is fresh water cooled and does not really get hot enough to heat the water. Besides, we don't cruise far enough from marinas and electricity.