What do you need the cargo carrier for? Can you store the items you were going to put in the cargo carrier inside the boat while underway?
Boats don't typically have tons of hitch weight... but clearance might be an issue as most cargo carriers I know of stick out 2.5-3ft from the rear.
I personally have a dual hitch extender (Dual Hitch Extender
) on my MH with a cargo rack in the raised hitch opening. My only use for it is to raise the cargo rack and extend it a little from the rear of the coach. The bottom hitch opening (the straight though one) has dragged before. I couldn't see coupling a trailer under a cargo rack.