The amount of current the boat hull carries. I can get a ground from the
hull, but I run a large wire to a terminal block with ground and power.

OK. I can get a ground anywhere too.
So are you saying that if everything is grounded seperately by a wire running
back to the battery it will flow through the wire and not the boat? Mine is
wired this way, but a lot of the electrical devices are mounted directly to the
aluminum and will work without the ground wire.

Is this because the wire has less resistance than the boat?

Is the object here to keep the current at a minimum, Im sure SOME of it will
always go through the hull just because of the motor and the electrical items
attached to the boat.

Is there anyway to test how much is going through the hull and what is
considered a safe level?