I have a 20hp go-devil that runs at full speed when out of the water but bogs down to less than half throttle in the water and it starts backfiring a little after a few minutes of running. what is causing this? need some answers quick. i already brought it to the go-devil place once 2 weeks ago and its not going again unless their work is warrantied.

Sounds like it is runing on only one cylinder....

Try removing one plug wire at a time while running - bet you get no change
when one is removed and engine dies when other one is removed...

Cylinder where there is no change is mis-firing or not firing at all...

take your carberator apart go to auto zone pick up b12 carberator cleaner in a gallon soke it for a day after soaking spray through all jets and put the carberator back on

Honda I will assume, clean the fuel cut off solenoid on bottom of carb.