Whatever you decide, it has to be one piece.
maybe I have been using dry suits for more years than many, diving,sailing,water skiing,windsurfing,since 1976, and one piece is crucial,
otherwise you get the 2 halves separating leaving a very cold gap up your back, brrrr!
Thick ones reduce mobility, good for diving.
Medium thick ones are good for dinghy sailing, and water skiing.
Thin ones that stretch a bit are good for Windsurfing, Ttips and Foiling.
I use the gull one piece, thin and stretchy, looks silly if you walk around in it, as it fits like a Lycra one piece.
it was developed for the 2012 sailing Olympics I was told,
brilliant bit of kit, I have 3 of them.