I'll respond to this as a diver, NOT on the subject of mines.
Movement - 3 or 5 won't make a jot of difference, they are both fine to wear on the surface, and a 5 will keep you a lot warmer for longer in the water (or when you get back out)
In a 5mm wetsuit, water temp 20 C, it starts to get a bit chilly afyer 30 minutes or so, BUT - the average water temp in mines is 6-7 C, I wouldn't consider going into Dorothea (v deep) or Vivian (we take trainees there) unless I was wearing a drysuit. Vivian I've done in a drysuit at a water temp of 5.5 C, and couldn't feel my hands for the best part of an hour afterwards (drysuit, insulated layers, gloves, hood, etc).
The only problems with drysuits - you will cook in it when out of the water, and they are incredibly expensive (oh, and if you don't know what you are doing in a drysuit, you can get an inversion, which means you drown with your feet in the air)
An alternative would be a semi-dry suit. I've never used one, so don't know how good they are.
Please note - all this comes from experience UNDER water, which saps body heat at an amazing rate. IN water (part-time) will be a lot different (as Vanoord has posted already)
Good luck !
Madness takes its toll, please carry exact change