A good boat refrigerator should keep frozen foods frozen and other products cold, but unfrozen, for days or weeks from an available energy source. There are normally two sources of energy on a boat: the ship's electrical power; or power from the engine.
You should match your refrigeration needs and desires with the energy available as well as the box's size and shape.
In order to select the correct refrigeration system for your boat and your needs, you should give careful consideration to several factors:

• The size of the box and amount of insulation, which can be evaluated by determining
  • the type of insulation
  • the interior size or cubic footage
  • size and location of the lid opening
• Boxes of less than three cubic feet are usually good candidates for 12-volt refrigeration, although larger boxes can still use 12-volt systems if the insulation and/or the DC power system is upgraded.
• Boxes larger than six cubic feet are normally good candidates for engine drive systems.

A good general reference book on 12 volt systems for marine refrigeration can be found Here.