As with most all things boat related the answer to your question is it depends...
Here in FL, the recommendation is at least 4" of foam insulation on all sides of the ice box. Less insulation, and it will be less efficient, and draw more power. So before starting you definately need to have an understanding of how much insulation you have and if you think you need to add more.
The next major decision is 120v or 12v system. There are pros and cons of each option. We have a 120v system on the boat and for our use which involves weekends and a few weeks of trips each year it works well. We like to leave the fride on at the dock all the time so it is always cold. So, since the boat spends more days a year at the dock, a 120v system makes sense, no reason to run the fridge on batteries when tied to shore power. When not on shore power 1 hour twice a day with the generator running and the holding plates stay cold enough to make ice in the freezer, even in the hot FL summers.
Good Luck,
--Eric
