The reason why Jet Ski hire might not be so common in the Mar Menor is because of licensing laws.
I found this on the following website:
http://www.marmenorspain.com/water-sports/mar-menor-jet-ski.html
"Jet Ski hire in Spain is restricted by Spanish laws to abide to. The main law is that a licence is required to use a jet ski. Please note that all jet ski hire companies will deny the request to hire unless a valid licence is provided. Those who hold a valid jet ski licence will be able to hire and enjoy the flat waters of the Mar Menor lagoon following a brief introduction to the jet ski. You will also be made aware of its specification and capabilities."
I have also only been able to find the one company who will hire on La Manga