Don't rent a jet ski. Many renters have to pay for damage they didn't create. Some have to buy a new jet ski. It's an ongoing scam and the police are in on it, taking 20% of the money.
If you refuse to pay, you get attacked.
Shit like this happens all the time:
http://phuketwan.com/tourism/phuket-...ng-sign-18900/
http://phuketwan.com/tourism/tourist...pattaya-15857/
http://thailandjetskiscamt.blogspot.com/
http://www.phuket.com/phuket-magazine/phuket-scams.htm
Also watch your motorbike if you rent one. They scam a lot on those too. ALWAYS lie about where you are staying so it's harder for the bike owner to find their bike so they can't steal it from you while you sleep.
Other than the scams, it's a fun place to holiday.