It looks like great fun, riding on the water on a jet ski. Surely, you will want to try; however, it may turn out to be a very expensive experience if you decide to rent a jet ski at a popular tourist beach in Thailand.
Your travel insurance will probably not cover an accident; partly because it is deemed an extreme sport, which they usually do not cover, and partly because you need to have a license to operate a jet ski. No license, no cover; not for damage to the jet ski nor for damage to yourself or other people.
However, that is not the only reason why you should not rent a jet ski. Many of the jet ski rentals are run by thugs that force tourists to pay for old damages which were already there before they rented the jet ski.
Typically, the scam is performed if a jet ski has an old damage. You are in a hurry to get out on the water to have fun, thus you do not see the existing damage. In addition, it can be impossible to see the damage if the jet ski is already in the water.
When you return, the owner hauls up the jet ski and spot the damage. You are then required to pay an amount for repairs and for the loss of income while the jet ski is in the repair shop. The amount can easily be as much as 80,000 baht.
Threats of violence
Besides being costly, it can be a very intimidating experience; the lessor may threaten you with violence or even firearms until you pay. While there may have been one or two Thais present when you rent the jet ski, there is suddenly a lot of Thais when you have to pay. It can be a very frightening.
You cannot expect much help from the police. At worst, they cooperate with the rentals because they get bribes. Even in the best-case scenario, they cannot help you avoid paying because it is impossible to tell whether the damage was there beforehand or not.
However, it may still be a good idea to get hold of the police. If nothing else, the police can guarantee your physical safety.
The tourist police can probably provide better help than the ordinary police; usually, they are not as corrupt and they speak foreign languages. The tourist police or the ordinary police can perhaps also help negotiate the amount if the claim is too exorbitant compared to the damage. Read more about getting help in Thailand.
There may be honest jet ski rentals in Thailand, for instance, I have not heard about scams on Koh Larn. However, it may be better to stay away from the jet skis altogether, as it is better to be safe than sorry.
Please note that you cannot rely on the recommendations from other tourists, as it is not everybody that will be requered to pay for an old damage and you have no way of knowing if you are the unlucky one.