Battery Charger / Maintainer
You are on the right track in looking for something that won''t boil away your batteries. A good choice would be "Smart" multi-stage charger like a Xantrex Truecharge. Although that particular model is not waterproof, there are others out there that are. I have not ever had good luck with Guest Chargers. The last, and I mean last one I had was to replace a Guest that failed, but it was an old style fry your battery type that came with the boat. I replaced it with their new model. Three years later the new model fried a circuit board. I call the factory about repair (this was a $400 charger) and they are very flip in telling me they don''t make that model anymore nor do they repair it and to buy their once again newer model. I went with the Xantrex.
Also on the battery, normally I would suggest twin 6v Golf Carts for the house system, but I think by your description that you have a boat without a lot of systems like refrigeration, inverters, etc., so just get a deep cycle group 27 battery(not a dual purpose one) , battery switch, good cable, and you should be good to go.