Boat Troubles

I'm having a problem with my 25' Sportscraft I/O and wanted to know if anyone has had this problem. When the boat starts out in the morning its runs like a champ going about 25mph at 350-4000 rpm's. After 15-20 minutes the speed and rpm's both drop way down. I went into Canada and by the time I hit the line I dropped to 2500 RPM's and 17 mph and then to 2200 at 14 mph??? The Fuel Filter and Water Separater are both clean? Gas pumps in fine without backing up and shutting off the Pump so it must be vented OK? Its a I/O with a Edelbrock ? Carb. and a Bravo 3 260 HP Merc.
Any Ideas?

Do a compression check as soon as possible it is rather easy to do! Could be leaking compression at warmer temps and not at idling speeds or cold. I am in the middle of deciding how I am going to approach a motor replacement because of a blown head gasket that torched a channel in both the head and the block between cylinders 3 and 5. Ran good the weekend before and like crap the next weekend wifey claims that we were not getting the speed out of it towards the end of the season last year before this happened I didn't notice it myself because I only run it WOT for short periods of time when returning home, but I now believe her.

Thanks for your input, I'll have that checked out.