Hi Von - I went there last year for a mud massage and it was lovely! They give you a pair of paper knickers and then massage the mud all over, wrap you in towels and then leave you to relax. Then you are hosed down to get all the mud off - the only thing was that it was a gorgeous young man who did all this - bit much for my high blood pressure (but I loved it!!).
Then we went into the pool and they have lots of water jets, steam rooms and jacuzzis etc. The cost was 55€, not cheap but we did feel really pampered. Ended our session with a nice meal in the cafe looking out over the Mar Menor - highly recommended!
I think we booked but the place was pretty empty so it isn't really necessary. As Noelaspill said, there are different choices for treatments so you may want to visit first and select something in advance.
Do let us know how you get on if you decide to do it!