Seaward Products is a major manufacturer of marine hot water heaters.  All heaters feature aluminum hold down brackets, ignition protection and your choice of marine galvanized or stainless steel exteriors.  Models options are : 6, 11, 18, and 20 gallon capacity, horizontal or vertical tanks, front or rear engine heat exchangers.  Check with your Boat Electric representative for the model that best suits your needs. And, do not forget to ask for all of the items you will need to complete the project such as;  hoses, clamps, fittings, and electrical connectors!