I have three "wetsuits" for Scuba and Spearfishing.
Classic Wetsuit (4mm)
Free Dive 2 piece (5mm & 7mm
Semi-Dry (7,6,5mm combination)
Classic Wetsuit, is great in moderate to warm water where you just want some extra protection and warmth. In my opinion anything over 5mm and its time to start thinking about another option. The extra warm isn't worth the movement restrictions and while they can work in the cold its not where they excel.
Free Dive 2 piece, now sees the least use for the simple fact you have to apply a wet lubricant to get it on. They fit super snug and are much warmer than your basic wetsuit and are cheaper than a Semi-Dry. Once you get to 7mm thick they really start to restrict your movement and are much harder to remove.......Honestly, when it comes to Scuba specifically I would avoid one.
Semi-Dry, I think of it like a Wetsuit with seals to slow down the rate at which water flows past the body. Compared to the other two options they are IMO superior in moderate to cold conditions and are an excellent comprise if a Dry Suit is out of your each. They are a little more tricky to put on than a Classic Wetsuit, but much easier than a 2 peice with the added benefit of not having to apply a wet lubricant.
Personally, I don't mind getting a little wet when I dive. However, when its relatively cold I dislike the rush of cold water from a Classic Wetsuit or the procedure of putting on a 2 piece . The Semi-Dry is the perfect solution for me in winter. However, its never going to be a dry-suit and i really don't feel they are comparable products other than the fact they are both classified as exposure protection.
