I have a Lavacore full skin and a Henderson 17 oz. polyolefin skin that I wore under my 7/5 wetsuit before I got the drysuit. I like both of them. They are neutrally buoyant and don't add to the constriction on the chest that another layer of neoprene does. They say they are equivalent to a 2mm suit for thermal warmth. If you are diving in cold water, the suit openings [ankles, wrists, neck] are sealed [booties, gloves, hood] anyway so there should be no water flow/circulation through the suit. I never did understand why the neoprene wetsuit has to touch the skin anyway. A trick that I use is to drive to the site with the undergarment on and with the heater blasting in order to work up a sweat. By the time I get to the dive site my undergarment is damp all over and I don't need to pour water down the wetsuit. You can probably wear poly or wool or some other synthetic but I think they might add to your buoyancy charcteristic. It depends how much air is in the fabric threads and the weave. I also wear the Lavacore under the drysuit and it works great [with a few other layers of polyester on top.] Eventually you are going to want a drysuit where you live if you get the diving bug.
