performance problem with 500 thunderbolt

[grew153owner]

I have a 1967 mercury 500 thunderbolt that I recently bought on a 15 ft grew fiberglass boat. It starts on the first try and idles fine, the problem is on the water I can only get about 7 or 8 miles per hour out of it. Seems like it goes to about 1/4 throttle and stops......i can throttle it all the way forward with no increase in rpms or speed. Out of the water with muffs on everything seems great. Can anyone tell me what I should check first. I have never checked compression or timing ( I don't know how and would have to bring it to a professional....which is probably a good idea anyway). Could be as simple as changing the fuel/air mixture?