I would check the prop first. Could be growth or fowled.I had the boat out of the water a few months ago. Hull was fresh then, and prop was clean. Engine is a Universal M35B, not making any smoke. I've had the boat about a year; this is how it's always been. The throttle is not maxed out....could a tachometer really get out of whack? How do you check that? Other people with the same boat/prop get a lot more speed.
"a few months" can be a long time. I had my bottom cleaned last august, the diver checked my prop and zinc as well. Said everything was fine. By November when I hauled for the winter my prop was all but gone from electrolysis. I'd post a picture of the prop, but I don't want to scare you.