I have known guys who wore nothing under their wetsuits ( going commando), and also those who wore a bikini styl speedo (bad choice for most of us), dive trunks ( briefs size tight fitting sawim suit, and some who try to crawl in a wetsuit with their bulky jams on. My favorite is a boxer type swim suit speedo used to make makes for training- light material, not bulky, but I don't see it on their current web site. A brief style suit, as in water polo or competition, works well, as do the compression shorts.
DivemasterDennis