Water temp was quite warm, around 29-30.5C when we dove.

They offered a wetsuit but it was a bit thick, either 3 or 5mm, so I just wore a long sleeve rashguard and bathing trunks, as did a couple other male dive masters. That was perfect for me, but I have a few extra kgs of insulation.
All the women and most of the male customers wore wetsuits. Given how some of them were not exactly proficient in avoiding the bottom, rocks or coral, I suspect they wore them more for protection than maintaining a comfortable temperature.

When snorkeling I used the rashguard (for sun protection mainly) and shorts again. My wife wore a 3mm wetsuit since she gets cold easily. We were out for over an hour and she was fine. That water ranged from maybe 27-31 depending on depth and time of day. In retrospect I should have worn gloves, as the Yonehara maze at low tide had some pretty tight spots and I did nick my thumb. Coral cuts take a week or two to heal.