Here is some information about wet suits, including a temperature chart. Hope it helps.
Wetsuit Thickness Guide and Temperature Chart | evo
I went snorkeling on the Great Barrier Reef in June. the first stop was without wet suit. Despite fairly warm water, I got very cold from being in the water so long. The second stop, I rented a wet suit from the snorkel/dive outfit whose boat we were on. there was a big difference in comfort level. I'm sorry I don't know the thickness of the suits.
My very, very thin d-i-l used a half suit (top) covered with a whole suit to stay warm.