I have a '06 90 Optimax on an 1800 lb Seafari, and cruise at 25 mph, and top out at 33 mph
(now that I've got it propped right). At cruise I get 5 mpg.
In the early 1980's I sold a number of new 20' SeaCrafts rigged with 85 and 90 hp Johnson V4's.
Finally, how much power is enough? Out in the Gulf of Mexico, even when I had 150+ hp on this same boat, I generally cruised at about 25 mph or so. Now I still cruise at 25 and use about 40% less fuel than before.
90 hp is enough, except for slalom skiiing.
__________________
Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury