Just back from Akumal. My computer read 25 - 26 C, divemaster's 28 C. That'll be 77-82 degrees to you. Buddy had a 3 mm full suit, I had a 3mm shortie over skinsuit, both of us ended up adding our hoods.
Snorkelling in one cenote was COLD. If I wanted to dive in water that temperature, I'd drive down to Brockville on the St Lawrence. Jeez.
If you can switch from the AI's dive shop to Blue Experience (whose shop is in the Grand Sirenis complex), I'd seriously recommend it. What a fabulous operation they are. They'd certainly rent you a suit that works for you.