Hi - we have a small air cooled Frigoboat unit too and have used it for several years - so lots of experience. And same problems....
What we did to reduce amperage use was:
1. Remove the ice cube trays and put a plastic bag filled with water into the condenser unit. This froze and basically turned into a 10-lb block of ice - which really kept our small icebox
very cold all the time - and reduced the running time/amperage use.
2. Removed the small fan assemble and wired in a 6" Hella fan to blow cooling air over the entire assembly. The Hella fan draws the same amperage as the tiny factory fan assembly but moves a much larger amount of air.
We have since replaced the first unit with a slightly different Frigoboat unit (one that is laid out in a horizontal design vs. our original "stacked" design) and opened up the area for even better airflow.
Instead of using a plastic bag filled with water, we now have filled the icecube tray area with the smaller size "blue ice" bricks.
The first unit we used for the 6-years, including 2 while we were in the Carribbean. It was always left on. We eventually bought a Wind
Bugger to help keep the batteries charged and found that with the modifcations mentioned above and with the Wind
Bugger producing power were able to go 60 to 90-days at a time without having to run the engine
We really thought about the water cooled option when we replaced the old unit - water temperature is always going to be colder than air temperature in the tropics. Supposedly you do not need a pump to circulate the coolant
(the motion the the vessel is enough) but I think that you will still have quite a bit of heat coming from the compressor - and may still need a fan to help with cooling.