Mexico is similar to most airports in not allowing any liquids onboard (except duty free in sealed packages and delivered directly to the aircraft door), beyond the plastic baggie with 100mL-or-less containers inside. They do a 100% physical search of all carry-ons on the jetbridge to ensure compliance.

It's not really a big issue for me departing from Mexico for a 2-3 hour flight max to the States, but when departing from, say, SCL or EZE (with a 9-10 hour flight ahead of me), I find it incredibly irritating that I cannot bring my own bottle of water, even if I filled it inside security.

I would recommend carrying a doctor note explaining, in both English and Spanish, the medical need for water with which to take prescription medication, or for any other specific condition. The airlines cannot take away your water if you have such proof.