One thing about properly fitted Underarmor tops: they are somewhat constricting toward breathing and ... internal organs. This isn't too noticeable, immediately, and easy to write off, until one wears a top for a long while - as I have done under my business attire.
Something a bit looser - the "Starter" brand of t-shirts that wick away moisture (Dri-Star, I believe, sold in Wal-Mart) would be slick, they are just as thin, dry just as fast, and aren't made to be as tight as the underarmor. Of course you can buy one size smaller and, I believe, still not be constricted at all (compared to an underarmor shirt).
I only bring this up based on that it does place an additional load on breathing when breathing comfortably may already be an issue underwater. Try both brands of shirts out for hours on end and you may see what I mean.