We have AT&T and were just in Cozumel on the magic. We only stopped at Cozumel, but we used our phones all day while sitting on the ship. Since AT&T covers Mexico, you just use your phone like you do in the states. You use your normal plan and data. We did not use WiFi, we used the data in our normal cell phone plan.
We never left the ship. We turned our phones off airplane mode, had to enable roaming and then our phones said AT&T Mexico. We just used them as normal.
When we got the bill, there was a separate section for phone calls, messages and data used in Mexico. There were NO additional charges. It was just noted separately.