For warm water (25-30C/77-86F) dives in the drysuit (redundant buoyancy) I wear a compression layer, the ones at the sports store made for workouts and running are cheap and effective. I originally ordered these and then realised they were identical to my existing undershirts.
I have a set of the Seaskin stuff and have a second set on its way here now. None of my Under Armour is quite as warm or wicks quite as well, to me. I may not have the right UA stuff, though. Regardless, the Seaskin stuff is a lot less expensive than UA. I mean, it almost qualifies as cheap!