so why are drysuit undergarments any different than skiing attire? what's a "pro" undergarment?
You can use the longer stuff from underarmour, but it's still designed for more "active" sports. Some will say scuba is active, but in the grand scheme of things, it's really not. The thing with the cold gear is it's primarily designed to wick to help keep you warm. The fibers have some insulating quality, but it's designed to wick. Which is great for certain parts. I still wear UA boxers. They're longer stuff is not primarily aimed at keeping you warm. The REI stuff does that, it's wicked thin, and is truly a base layer, not primary insulation. For some, it's warm enough, for others it's not. It's identical to the Xerotherm baselayer, but much cheaper. For above mid 60's, you won't need anything else. Once you get into 50 and below though, you want to have the Arctic, there is simply nothing else in the market quite like it. For the price, it's quite cheap, but rather unnecessary unless you're doing wicked cold water.
For the caves, there's no reason to spend twice as much for any "branded" stuff that is going to be the same as the REI stuff.
So back to the OP. There is no science behind the drysuit undergarments. They have some nice added features. Stirrups on the ankles, thumb loops, and crotch straps. All of which can be found in ski gear. No reason the ski gear can't work for drysuit undies. I hate jumpsuits, mainly because they're impractical. They're huge and bulky, and don't promote layering, which if you look at the cold weather people, everything is about layering. Can adjust your thermals easily, and instead of blowing $600 on one jumpsuit, and find out it's too hot, your sol. Layer, and you just add or remove layers and your good. Cotton works fine, just not when wet, so it's not ideal, Polartec/thinsulate type stuff is ideal from a fiber standpoint. Using normal clothes isn't a problem as long as you chose the right stuff. The undergarments used in cold weather mountaineering/backpacking is what you want to look for as that is comparable to the activity level we are doing, and similar thermally.