I need some advice so I can fix this thing that is driving me nuts.

Here's the history, last Nov it lost 4 mph, was running 74 and it dropped to 70 mph but it seemed to run great other than the 4 mph i lost. Then in May I took it out and no change. So, it had 107 hours so I went ahead and did the rings and honed cylinders and cleaned carbs which were not dirty. Had 120 lbs compression on all holes and had the same after the rings. Sounded great in the garage, went to the water and didn't get out of the dock area because it wouldn't hardly take off, it might have gone 15 mph. Back home, took carbs off, looked great, fuel pump was clean, it had new plugs all along, took jet pump off and it was in great shape. Went out today, no air box so I could have room in engine compartment and it would go 47 mph. It didn't stumble or almost die when I took off, instead it was slow to rev but would slowly gain speed and get to 47 mph and 5500 rpm. Came home, fuel valved was checked and was clean, took exhaust off and it was not plugged, didn't take water box out but blew air thru it and seemed to be fine. Then I came here asking for HELP PLEASE.. God bless