Do they actually help keep your core warmer?
My swim partner this season uses a wetsuit regularly (sleeveless). On the occasion of a 10K swim he chose to also wear a thermal shirt; water temp was 64F-65F, not terribly cold but over a 6 mile distance can become quite chilly. Afterwards, he was shivering more than me, just saying. Could be because he always wore a suit and wasn't quite acclimated. How acclimated are you? Water temps?