I currently have a 29' Envision combo that I purchased with everything but the block. I dropped in a 454bbc with 781 cast heads, comp cam, edelbrock rpm intake and IMCO Powerflow exhaust. I have had the boat out once for trial run and could not get it past ~ 4500 rpms at WOT. I am shooting for around 5500. My first thought was to go from a 23p prop to a 19p prop but now my concern is that the lack of rpms may not only be the prop but possibly a rev limiter as well. I was told that if I have a knock sensor (which I do) then I have a 4800rpm rev limiter. Can anyone advise?

You would know if it was sitting on the rev limiter. What ignition are you running? What induction system? carb or efi? Give us a little more info on the engine.
Eddie