The biggest you can fit into the available space.
The bigger, the better. If you can fit in a group 29 or 31, go for it.
If you go with the group 27, you can add another one in parallel later, if you are running out of battery power.
No it wouldn't...you would have to put those 6 volt batteries in series to get 12 volts and your amp hours would stay the same.Ya, I'll see how one does. I don't want to carry around the weight of a 29 or 31, I use the boat more for tubing skiing more then fishing, and I don't want to lift in and out a 29 or 31. I suppose I could go with two 6v batteries, that'll give me more amps.