I purchased the Kokatat Polartec liner for my GMER drysuit and I have used it a few times while both sea kayaking at 38 degrees and surfing a sit-on-top in January with the air temperature of roughly 40 degrees.
As the previous reviewer mentioned, I felt the cold but did not feel cold. I also had a thermal top and bottoms on under the liner. I don't know if that made a large difference or not. The liner fits great, does its job, and is well worth the money.
I wanted to have something to wear under my Kokatat Super Nova Semi-Dry Suit to insulate me from the cold water, and wick moisture away from my skin. I didn't want two pieces of "long underwear", as I feared while I was paddling the top might ride up, or the bottom slide down under my suit, and I would not be able reposition it. A one piece wicking liner seemed the best for me.
My only complaint is that it started to pill really quickly and I have fleece pills all over the place.
I walked out into the water in my suit, and I could feel the cold water, but I didn't "feel cold". It did what I wanted it to do.
The air was about 50 degrees, and blowing about 10 mph, but the sun was very bright. The Polartec 100 liner wicked the moisture away from my skin, and I was very comfortable. I thought I would be sweating with the sun beating down on me, and wearing a life jacket. But I wasn't. I was far more comfortable that I thought I would be.
If you are looking for something to wear under your Semi-dry, or Dry suit, look closely at the Kokatat, Polartec 100 Dry Suit Liner. It does a great job.