It won't be the prop-nothing to do with propping will affect the pee stream unless the exhaust is severely restricted by the quicksilver prop.
I'm running a 17" pitch and probably need to go up to 19" btw but that's more about speed than pulling a skier out.
I'd do a compression test on it too but the only way low compression could tally with the pee stream stopping would be a cracked liner or leaking head gasket. Might be worth popping the head off, giving it a thorough examination and maybe doing a crude leakdown test on it.
If you want to have a chat to a proper Yamaha marine engineer it'd be worth going over to the iboats.com forums and asking the question in their dedicated Yamaha repair forum.
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