Probably an Atwood
http://www.atwoodmobile.com/marine-prod ... eaters.aspThe common arrangement is a heat exchanger with flow and return pipes from the engine cooling system and a 110 or 240 volt immersion element for shore power.
Before buying a replacement make sure it will go in without removing the engine or a number of engine attachments such as the port side manifold. The original would have been installed before the engine went in.
You don't have to replace it with the same make, also look at Isotherm and others. A stainless tank will probably last much longer than an aluminium one but stainless costs a lot more.
Scroll down about 3 pages (1st posted Aug 22, my photos added Jan 3) to "Water heater replacement on 258 Vista?" and there is a photo of my original Atwood and a replacement.
Some photos of the Atwood
http://www.ebay.com/sch/i.html/?_nkw=Bo ... %20heaters