motor problem or prop problem

[Soundslikefun]

My 1964 Mercury 65 hp finally runs quite smoothly. Always starts first turn, idles nice, etc. In the driveway this thing sounds like it really wants to get out and go. With the boat in the water it doesn't rev very high and I am lucky to hit 10mph. Yeah, 10mph. Boat is 16' MFG deep v hull. Same boat last summer hit 30 all day long with a tired Evinrude with no compression. Current prop is a 15 pitch. Not sure what the one was last summer. Does this sound like a spun prop (I don't think so) or some powerhead/drive problem? We have about 800 pounds of passengers in the boat and it does run better with just me, but we had the same passengers last summer as well. I think I am so close to having this all dialed in, but just missing something.

Any/all suggestions welcome.