First things first...
:welcome:to iboats. Great to have a new member join us...
As for your question about where to mount a circuit breaker on a raft. It you don't have any panels or such, then mount it at the battery itself. Some breakers have a tab on them to tighten to the battery post and then your wire will come off the other end of that breaker to go wherever you want. JMHO!