8hp kicker bogs out in water

[billy the bass]

Hello and Thanks for reading. I have a 2005 8hp Mercury hand tiller that runs great hooked up to the muffs but when I take it out on the lake and get the throttle about half way to max throttle it doesn't progress in speed. On this model you can press a button and rev it up in neutral and it sounds like it runs full rpm and getting gas but once you shift in gear and go full throttle it runs at id say 65% power.. any help would be greatly appreciated.