Carefully consider your safety and the implications of accidents if you hire a motorcycle or jet ski. Australian travellers continue to report harassment and threats of violence by jet ski operators on tourist beaches, particularly in Phuket. You should check with your travel insurer whether these activities are covered by your policy. You may be detained and arrested by police following jet ski and motorcycle accidents until compensation, often in thousands of dollars, can be negotiated between parties.
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Australian travellers continue to report harassment and threats of violence by jet ski operators on tourist beaches, particularly in Phuket. Many travellers have reported that, after returning hired jet skis, they have been confronted by gangs claiming that the tourist damaged the jet ski. There have been reported instances of such gangs threatening violence, including at knifepoint, if a large sum of money in compensation for the alleged damage is not paid.
Operators may request your passport as a deposit or guarantee before hiring jet skis or motorbikes. If there is a dispute about damage to a rented jet ski or motorbike, rental operators may try to keep your passport until they receive compensation. Passports are valuable documents that should be appropriately protected. We recommend that Australians do not provide passports as deposits or guarantees.
From:
http://smartraveller.gov.au/zw-cgi/view/Advice/Thailand