If you are really committed to buying a jetski, I think your best bet is to get one now. If you have to take out a loan for a few months until you sell the sled, it would be worth it.
The reason I say this is because everyone is selling their ski's for dirt cheap right now because the season is ending. In addition the economy isn't that great so people need money too so they're willing to let a ski go for cheaper than normal. When March hits (at least in places where the lakes don't freeze), people start to get their ski's ready for spring. March through May are the BIG buy seasons and that's when prices are the highest. I wouldn't take a low ball offer in March or April because someone is bound to pay full price soon. If someone offers you a $400-$500 less in November/December...it's take it or wait 4-5 months and hope someone still wants to buy it then.