A bathing suit for starters, a rash guard, or most surfers wear nothing under.
A common additional item to wear under a wetsuit is a one piece lycra suit. The benefits of this are that it makes the wetsuit easier to put on and off, and the lycra doesn’t compress. For instance someone diving a 5mm farmer john (2 piece) wetsuit and a 2mm lycra dives deep enough that the wetsuit has compressed to half its original thickness. This diver now has 2.5mm of wetsuit covering their legs and arms, 5mm covering their core (chest, stomach.) Now when you add the 2mm from the lycra which hasn’t compressed there is actually 4.5mm on the legs/arm and 7mm over the core area.
A diver in a 7mm farmer john at the same depth is left with 3.5mm on the legs/arms and 7mm over the core. If you double the depth at this point the 5mm and the lycra gain a lead over the thicker wetsuit.
The free aditional benefit to this item is that you may be able to get rid of some extra weight as you can use a thinner wetsuit which will require less weight to get you under.