Warning: I've never used an isolator so anything I say
about them is only from reading the catalog and thinking...

I think for your size boat & electrical needs that a small
battery combiner will be simpler, smaller, and cheaper
and still do the job. That's what I use. It nicely separates
the batteries so the "backup" one can't get drawn down
yet connects them when the altenator (or generator in my
case) is putting out charge - so both stay charged. My
1/2/ALL switch is now simple to operate - OFF or 1 is all
I ever need unless I run the main battery down.

Looking at the description of isolators it would seem they
perform the same basic funtion (separate the batteries from
each other) but would be needed when....

If your two batteries or banks of batters were of different
types, ratings, etc., you might never want them conneted
to each other. The battery combiner *does* connected them
when the system is charging (voltage over some level). The isolator
keeps them separated (via diodes) at all times.

Hope that is rignt and helps or at the least sparks more
conversation.

Gary