Firstly choose the right refrigerator: An eutectic refrigerator would let you put it in the Boat and run off the eutectic system WITHOUT power! This is more of a backup and allows you to run with no power. You can get 23 hours of operation before the temperature in the eutectic refrigerator will start to increase.

Secondly how to power it .

There is only one way and it is the best: A Fuel Cell.
Choose a relatively small battery pack that would be directly connected to the Fuel Cell. The Fuel cell will automatically come on and charge the battery. The 1600W Fuel Cell will comfortable manage 1 or possible 2 Eutectic Fridges.

Just set and forget...(replace fuel cartridges after every 2 weeks continous fridge use)