The mainshaft bearings in the outdrive (where the crankshaft hooks to) are a pretty good suspect. It turns all the time with the engine running . Ive seen several that had bad bearings and cause the loss of power from the engine when trying to operate the boat. When the bearings go bad , they can seize up,causing the engine to bog,also chewing up the housing as the locked up bearing spins in its bore.Not pretty.
It should be easy to check with the engine out.
Fact is stranger than Fiction