As you can see, you'll get a wide range of answers when you ask that question since everyone's comfort level is different. Once you get more dives under your belt, you'll realize the better question is "What's the water temperature?". But when you're first starting out, it's really a bit of a guessing game.
I'm warmblooded, so for Coz I'll typically dive in a 2mm shorty except for the summer when it's just a t-shirt and swim trunks. But I'll see others on the boat wearing 3mm or even 5mm full suits and often hoods. I'd cook wearing a 5mm but there's no arguing comfort. If you're cold you're cold, and vice-versa.
Keep in mind that the cumulative effect of repetitive dives is also a factor so you may need more thermal protection for the second or third dive of the day than you did for the first. For me, that means I may choose to dive without a suit for the first dive, then put it on for the second or third. It just depends on how I feel.
And remember it's easier to cool off underwater than it is to warm up so you may want to err on the side of too much suit rather than too little until you figure out what works best for you.
Have fun! Diving in Cozumel is a blast.