HAiko:Does it make sense to wear some undergarments under a wetsuit for insulation? I know it's all wet, but would, let say, a t-shirt (or more) be any comfort?
Yes and no. I tend to wear a nylon rash vest - more for comfort than warmth. It's a smooth material and the seams tend to be worn away from the skin to avoid rubbing.
If you're interested in warmth, have a look at this:
http://www.fourthelement.com/content.php?content=home.php
I wear fourth element gear under my drysuit and I know people who wear it under their wetsuits. It's claimed to be neutrally bouyant (slightly far fetched IMHO, but I do wear less weight in my drysuit than with a conventional undersuit) and makes a huge difference to warmth - around the same as another 3mm of neoprene. It's good stuff!