If unsure can one use a bigger gauge wire? Any problem doing this?

Not a problem at all, assuming physical space limitations allow it. Connectorizing the bigger wire can be challenging though.

Al, You must not be pushing those amps much If they are not even warm. Most of the time you could almost fry an egg on the amps in my cars. I have not had the boat stereo installed long enough to find out if Im going to heat it up.

How old is your battery?

In our last boat I ran two amps, 6 speakers and a sub and never saw the volt meter drop with 1 battery. I did run large enough wires and overbuilt the system, but never a drop, even when playing loud.

I don't think engine size, etc. makes much difference. My 2 cents is to put the (2) batteries into a switch (1,2,Both) and you can have the best of both worlds.