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I got five informative responses.  Two were actually at the Moon Palace
Resort last year and one was at a sister resort.  One stayed in a rented
house and one is planning to go to Cancun soon.

All said that the food at the resorts is pretty basic--not fancy-- with
a lot of corn tortillas, omelets cooked on griddle in front of you in
morning, grilled fish, plain burgers, etc.    All felt that it would be
pretty easy to eat gluten free.  All said that I would have to wait to
get there to talk to the chef and wait staff. (The resort has still
never returned any of my e-mails on the subject.)  Most of the staff
speak fairly good English.  If the one assigned to us doesn't, others
will come over and help.

Several mentioned to bring our own food and bottled water on the excursions.
Some spouses without celiac disease who ate the food provided on the
excursions even got sick.

A non-celiac issue:  I got mixed info regarding the water.  Most said to
only use bottled water and avoid all cut fruit and fruit salads.
Another said that they got a water filtration system at the Moon Palace
which makes this no longer an issue. I guess we'll check into that more
when we get there.

I'll write again when we return if there is anything helpful to report.