Engine wont rev past 2700 rpm in gear

Hey there! I am new here but certainly not new to boating. My 280 Sundancer is equipped with the V6's. They've been well cared for and serviced by my self and the previous owner. Just out of nowhere the port side engine decided it does not want to rev past 2800. Out of gear sitting in the slip the engine will freely rev all the way up with out issue. In gear, no dice. I have checked the cables. The throttle cable opens the throttle blades to full throttle (not running of course while I am checking). Any ideas? I am not new to engines, mechanicals, etc. but this one has be stumped.

Bottom Clean? Props Clean? I would start by verifying fuel pressure and doing a compression test

injectors

Of course many things can cause your symptoms. It would help in you provide some more info in your profile such as year of boat, what drives and your location. Additional info such as raw or fresh water cooling and if you have carburetors, EFI or MPI fuel systems. Otherwise we are just guessing and 80% of your responses will be meaningless.