I was looking at installing either a new inverter (original one was only like 600W and is 30 yrs old, doesn't appear to even work) or a combination charger/inverter. After looking at info on batteries in general I think I have some other issues and would appreciate any recommendations or feedback. This is on a 1995 440 aft cabin.

For starters, my boat only appears to have two starting batteries in the battery box. These provide power for starting the engines and supply house power. The web site gives no Amp-hour rating for the batters. Will this be a problem running an inverter off the starting batteries other than the fact I could run a risk of draining the starting battery? There are two other batteries - one for starting the generator and one for the bow thruster.

Should I split the batteries and have one dedicated to starting and get a deep cycle for house power? Or add batteries, although I have no idea where I'd put them?

There's a battery selector switch which was set a "Both" when I bought the boat so I've never changed it. Been reading up on how that should work.

Next, my batteries always have some water on the top. I know a few things can cause this, but my battery box is not ventilated at all. I'm also in a high humidity area (Houston, TX). I'm thinking about cutting a hole in the lid and installing some sort of screen, although I sit on the lid when working back there. About how much vent area is needed for that?
Thanks!