I always dive a full 5MM and wear a tropic beanie during the winter months these days (age has a way of catching up with lots of us). I also added a Lavacore vest last year while in Hawaii in Feb (78 degree water there at that time) that I can wear under my 5MM when necessary. Nice thing is it adds no buoyancy.
I'm somewhat convinced it's not so much the water is overly cold being 80-82 in December, it's that the air temp and humidity are much lower which makes for a colder morning ride out, a colder SI and colder trip back in. If you get a chill in the water, during the SI it seems to build if it is cool and windy and low humidity... Some will literally sit shivering under boat coats anxious to get back into that water that all of a sudden feels like a bathtub as the chill during the SI builds and we all know that even though it feels warm compared to the air and wind it's going to chill you even more on the next dive(s). Add multiple days of repetitive diving and that can take a toll.
Anyway, my goal is to not get chilled in the 1st place. I'll take too much heat over chill any day. One can always cool themselves in a suit that may get too hot by pulling the neck of it out and sucking/flushing some cool water down into and through it but there's no way to heat oneself in the water once the chill sets in.