You will want to put a 'riser' on your transom that raises the engine to a point so that about 2/3s of the jet 'shoe' are below the bottom edge of the hull. (how's that for a run-on sentence)

My other input would be on the size you are interested in. If you typically travel light, a 25 may work ok for you but it works out to be about 17hp at the nozzle, not 20. That may not sound like a big deal but it is. I ran a 40 hp, 28 at the jet, on my 14' footer and it was a nice combo. I think you are on the right track with what you are trying to do.

Steve