as Tim said its probably the impellor, but its worth also considering:-
A blocked intake as Tim said.
Sometimes the plastic grill thats fitted over the intake can be clogged if the boats been left afloat with the engine down.
I had a 55HP Suzuki, that when I first had it used to get its tell-tale outlet blocked with a limescale like substance. I used to carry a 6 inch piece of wire on board to poke up it. It stopped after a few trips out then went on for years OK with a new impellor each year.
Some engines, and I don't think this applies to yours, have the tell-tale pipe after the thermostat, so only 'pee' when warmed up.
But its most likely the impellor, and thats worth changing if you buy a new engine anyway. I change mine every year anyway, as I'm paranoid