Batteries loose a 1/4 amp a day to the air. just sitting, even when not hooked to anything. the lower the state of charge the more you'll loose and higher the humidity will cause more loss too. It can get up to 1/2 an amp. So there will always be room for some charge. Make sure you have plenty air room/flow around the heat sink on the charge controller, as when the panels
are putting out more than is needed the extra amperage will get dissipated through heat.
on another boat I had with wind
I used a load diverter type controller and ran the load to a heating
element in a small tank for hot water
. I had fairly hot water
most of the time, in the southern trades.