Try looking at the fuel injectors, I have just recently had to replace one of mine on my Yammy F115AETL 4-stroke as it had become blocked. I noticed that my WOT revs went down and the boat was noticably slower but didn't realise what it was until it was found by the engineer during it's latest service.
I'm yet to get favourable conditions coinciding with time out on the water in order to re-check my WOT and speed again, but the boat feels a lot better since changing it.
There's weather out there - must be time to RIB!
(Or dive, or ref rugby, or.......)