1986 regal ambassador 255 not getting up to speed

I just bought a 1986 regal ambassador 255 and have been doing work to it. I have a problem with it getting up to speed. My fishing partner is a auto mechanic and has been helping me he says the motor is strong but we cant get it up to a cruising speed. It seems like the boat wants to go but doesnt. wondering if you had any suggestions for me. Right now it will get me where I'm going just slow.