In recent years I've echoed a number of cautions regarding the rental of jet-skis which can be found in large quantities along the majority of Thai beach resorts.
I have finally come to the sad conclusion that the majority of the vendors renting jet-skis are nothing more than scam-artists that prey on vulnerable tourists.
COMMENT: In the majority of cases, renters of jet-skis cannot possibly know of every feasible mechanical problem that can occur on a Personal Flotation Device (PFD), which can easily be manufactured by someone knowledgeable on the maintenance of jet-skis.
Unless you know a jet-ski operator very, very well, my instinct is to avoid renting jet-skis. Period. Save your money for something far more practical.