Running on 1 battery VS 2?

[Bfun220]

I installed one of these on my boat and I won't install it without something protecting the wire. Never heard if you run a wire short enough that it doesn't need a fuse/breaker. Short wires can still overload, get hot, melt stuff and cause fires.
As for sizing, here is a chart with what they recommend.

Yeah - you are right do it correctly the first time. I was trying to get cheap as the fuse and fuse holders cost more than the switch and ACR.

I used a breaker instead of fuses. When I put the boat away for the winter I open the breaker and plug in the battery charger.