But he said all cylinder compression values were very close. If they
aren't leaking down, then how is that water getting in?

This is something that I have and are still considering as a possible
problem. I called a local OMC repair shop and that's what they
suggested - that when the engine gets warm enough and with enough
"suction" on the down stroke it could draw in just enough water (from a
'partially blown head gasket') to affect how it runs under load but not
enough to effect it otherwise. It's a possibility, however I would
think that there should be a way to tell if that's happening without
removing the head(s) and replacing the head gaskets. On the other hand,
that's not really a big, expensive or difficult job so maybe it's
worthwhile doing as a precaution.

JM