Go to your local DIY store and hunt around for a copper cylinder of the size you require, you need it to be fully insulated (sprayed on expanded polyurethane) and a bung to accept an electric
immersion heater, when on shorepower,(use oncalloy immersion heater for all water types) and a indirect internal coil to connect to your engine to heat the water. A marine
one was averaging £350 plus, the one from a local DIY store plumbing
section ? £55 ! fitted in 1988 and still in service
...if posible make sure the top of the tank is level with the engine to prevent thermal convection.
Hope this helps..