You should only rent from licensed Jet Ski operators and their licenses should be available for inspection. You could be fined for renting from a vendor without a license. Anyone operating a Jet Ski in Phuket is also supposed to hold a 'Second Class Helmsmen of Power Driven Sea-going Vessels' certificate. If you have not got one (which we can assume most people do not!) there is seldom a problem. However, if you have an accident it is more serious. Operating a Jet Ski without such a certificate means if you have an accident you could pay a fine of 10,000 Baht! If you are worried about this, just go on a Jet Ski as a passenger. Phuket law dictates that if you travel on a jet Ski even as a passenger you must were a life jacket that has a whistle-not a bad idea anyway really! In addition, according to the law, you should not go further than 500 metres away form the coastline-again, pretty much commonsense. Likewise, if sea conditions are such that a red flag is raised to people stop swimming, you are not allowed to use Jet Ski...again, a very practical law which most people should be happy to follow.