I used to go on tropical trips and dive in only board shorts. Not even a t-shirt shirt.
Coming from Southern California, where water temps are in the mid-50s a lot of the time, any water over 75f felt very warm to me.
And I really loved the feeling of diving with almost nothing on.
Then I noticed that on multi-day multi-dive trips, I started getting sick about half way through the week. Nothing serious, just typical things… stuffed up nose, sore throat, mild headache, just minor stuff.
It didn’t stop me from diving, but it was annoying.
It happened on several trips in a row.
I mentioned it to a diving friend with a little medical knowledge, and she told me something very wise, “Your body temperature is 98 degrees, and you're spending several hours a day in an 82 degree environment. So, the cold is putting stress on your immune system, which weakens it’s ability to fight off illness. Added to that you’re flying on planes, living in hotels, eating in restaurants… all great places to pick up new germs.”
So, since that moment of revelation I’ve always worn some kind of protection. Now I wear at least a 2.5mm shorty for shallow reef diving, or a full 3mm if I’m diving deeper wrecks (warmer and better abrasion protection).
And since making the switch to a wetsuit I've never gotten sick on a dive trip.
I know there are people who will read this and say they don't wear a wetsuit and never get sick.
Good for you. I guess we're all different, and this is just what works for me.
So just perhaps... it's not about how warm you feel in the water that should determine what you wear.