I bought a used boat (Four Winns Liberator 211) and I have noticed an
odd behavior. When I am in shallow water and I attempt to accelerate,
the engine revs but the boat does not accelerate like it should. It
feels like the prop is not spinning like it should be for the given
RPM, and then it may or may not catch allowing the boat accelerate
forward. When in deeper water, this never happens. This is only an
issue because I live on a channel and have to go through shallow water
to get to the lakes. Is this normal or any ideas on what might be
wrong? The shallow water is muddy but I don't believe there are any
hard obstructions (rocks, etc.)

Thanks for any help.