It is unlikely that the hirer will be allowed to contact the police to assist in the situation. Even if able to they are unlikely to be of any assistance.
Some noise has been made to clean up the business but as things are in Thailand they are probably not going to change much. Some people recommend that you should never hire jet skis in Thailand. Others have done so and not had problems. My opinion is that it is not worth risking your holiday and would stay away from them.
Here is a Youtube video showing this scam in action: CAUGHT ON VIDEO: Thailand jet ski scam exposed!!!