losing power and RPM' s any idea why

[explorer 542]

I have a 2004 115hp 4 stroke EFI Mercury outboard engine it has very little get up and go out of the hole works it's *** off to get the boat to plane might get 4000 rpm at take off with rpm picking up to 5000 with engine trimmed all the way up. I think this boat was never a rocket out of the hole but it is getting worse as the season goes on and I am losing WOT rpm too. I have changed the plugs checked the fuel filters and all seems good. I have no problem taking the boat somewhere to get it looked at but I use it everyday cause we live on a island and I can't be without it for the 3 weeks to a month that all the boat shops in my area seem to take. Any ideas injectors, stator I don't know does the engine have to go on a scanner of some kind.