I used to wear a skin under my suit and still do when I'm wearing my 6-7mil winter neoprene, but for a lightweight suit, trunks will do fine. You want to be able to take off your neoprene after the dive, as it can suck the heat from your body if you're in the wind or overheat you if you're not.
And no matter what FLLDIVER says, I do NOT wear a dive speedo (yet!)
And no matter what FLLDIVER says, I do NOT wear a dive speedo (yet!)