2-stroke marine engine (200 Yam) starts and revs just fine out of gear. Put into gear and will only achieve 40kph or so.
Old petrol (50:1)? Changed tanks and new fuel - same.
Boat hull dirty? A little, but exactly the same 6 months earlier, pulled 80+kph.
Heads off, no tracking on gaskets, but some emulsification in a couple of bores. Rings?