So now you don't need any switches or fuses or links, back to square one...
I wouldn't even worry about an isolator just disconnect your battery if you're not going to use the boat for a long time. The more stuff you've got the more chance of it failing.Put an inline fuse near your battery on the positive that goes to your dash.
Connect all the positives & negatives at the dash in 2 cable connectors retaining the inline fuses for the radio & fishfinder if they have them.