Almost certainly the insulation
on your boat is insufficient to gain high energy efficiency with a cold-holding plate or other conversion. If you have doubts you can make an ice melt test which is easiest to make when the ambient temperature is relatively high, like 80 deg. F or more. Do an internet
search on this test.
If the insulation
is not good then one removes the liner and all of what might be in there for insulation and put in a good vapor barrier, proper insulation and a new liner. More than likely you will have to add more thickness than your old insulation had which means that the resulting interior
dimensions will be less and a new liner must be fabricated.
There are several good sources for advising just what insulation you might opt to buy. The difference in battery
in warm weather
can be huge when you do a good job on the insulation.
I converted one box which had an Adler Barbor type of cooling
system. In the winter it seemed to run more than desirable. In the summer it ran ALL of the time and still couldn't keep the inside as cold as it should.
After putting in good insulation and a cold-holding plate I used only 13Amp-hours per day in the tropics to have good cold inside.