We use Frigoboat. Uses a sealed Danfoss compressor and no other moving parts (hence reliability). Heat is disipated through a bronze plate/block mounted outside the hull. Runs off DC. Uses about 2.5-3 amps average with outside water temps between 55-87. Keeps freezer at 20 +/- 1 degrees fahrenheit.
Secret is in the freezer box design and construction and use. Top loading, plenty of insulation, insulating blanket between lid and food. Keep it full of frozen foods/liquids (stabilizes thermal fluctuations). When we want to limit amount fo food at risk we put in blue cubes to provide the thermal mass.