Can someone explain this?

[TownsendsFJR1300]

we all are
we can all have a merry christmas and a happy new year now

+100 ^^^^^^

[zenoahphobic]

To add, I many years ago, was looking at jet ski's to purchase (Yamaha of course).

Should you tow this particular one (and the one's I looked at), you have to pinch off a water line (under the hood) or YOU WILL push water up into the engine.

I don't remember if it was a two or four stroke engine, but I suspect water would probably be pushed up the exhaust.

Yes
I think the problem is related to the fact that the exhaust is higher than the engine (unlike outboards where it is significantly below in comparison), and without the normal exhaust gases pushing the water out, gradually the exhaust will fill with water and gravity does the rest.

Someone should do the torque calculations that a prop in reverse can produce before it hopelessly looses grip through cavitation and ventilation. Although plausible when compared to the pressure felt to put a hand (as Boscoe said) or paddle in the water at even slow speed, I still think it extremely unlikely to happen because of, as I said, the massive "turbulence" occurring in such a situation!