As others have said, that's a tough question to answer since there is so much variation in susceptibilty to cold. Also, how much diving you are going to do plays a part - 3 or 4 dives every day for 5 or more days in a row is going to require greater protection due to cumulative loss of body heat. I just got back (last Wednesday from a week in Cozumel, in which I did only the 2-tank morning dives for days 1 and 2, 2-tank morning plus a night dive on Day 3, took a day off, then 2-tank morning dives on days 5 and 6. I was comfortable in just my fleece-lined 0.5 mil neoprene full-body Henderson. If I was going to dive more than 3 days in a row, or more dives in a day, I would step up to a 2 mil wetsuit, or a 3 mil shorty over my Henderson. FWIW, I carry a lot of my own insulation with me, and I grew up in Wisconsin where we thought nothing of jumping into a 68 degree lake, so I'm probably less susceptible to cold than most. There were two guys diving in our group that dove morning dives 5 days in a row in just swimsuit and teeshirt; I have done that before, and was fine for 2 days, but by the third day I was shivering. Most of the women in our group used 2 mil wetsuits, a couple used 3 mil plus a hood.
I do recommend full body protection, even if it is just skins. Hydroids can be an issue if you get close to the reef or the sandy bottom, and, while I saw no jellies at all on my recent trip, I have seen small thimble jellies on other trips, including several trips in March of years past when there were a lot of them on certain dives. These are not the dangerous box jellies of Northern Australia, but ALL jellies are venomous to some degree, and I wouldn't want to swim through a mess of them without body protection.