Also remember that there are several places right on the beach in Cozumel where you can get a massage. For $1 per minute with a 20 minute minimum you can get one at Paradise Beach.
I have never had one there but everyone that has posted about it says they are good.
Also remember that Carnival does not own the spa on board. It is operated by an independent company that simply rents the space. (just like the gift shops)