I recently had an opportunity to travel to Mexico for some spanish lessons.  I wanted to put up a little write up here so if someone else was searching google for some options they might come across my article.

I arranged for lessons with Francisco Romero of the Cancun Spanish Language Institute.

http://www.studyspanish-mexico.com/

We had a great experience with Francisco.  We opted for an immersion type experience, with Francisco speaking almost no english during our lessons, but unlike some of the other instructors we have had, he was clearly 100% fluent in english, and could help explain some of the nuances in english. 

I would really recommend him to anyone else considering taking language lessons while in Cancun.