I used to dive an Aquala "dry" suit (it never was completely dry) over a wet suit's farmer John pants. That is quite a good combination, and I stayed warm when others did not in wet suits.
Today's dry suits are of two types. Some require that underware (if a trilaminate) be worn, as they are just a waterproof shell. Others, made of foam neoprene, are buoyant themselves, and excluding the water provides some of the insulation. Adding underware to the foam suits increases the amount of insulation, and therefore the buoyancy. You can loose that buoyancy on either of these suits if the underware is soaked with water. But all you need to do to get back to neutral is to drop the weights you carry. The water inside won't affect your negative buoyancy, but displaces the air, which is quite positively buoyant, as stated by others above.
SeaRat
Today's dry suits are of two types. Some require that underware (if a trilaminate) be worn, as they are just a waterproof shell. Others, made of foam neoprene, are buoyant themselves, and excluding the water provides some of the insulation. Adding underware to the foam suits increases the amount of insulation, and therefore the buoyancy. You can loose that buoyancy on either of these suits if the underware is soaked with water. But all you need to do to get back to neutral is to drop the weights you carry. The water inside won't affect your negative buoyancy, but displaces the air, which is quite positively buoyant, as stated by others above.
SeaRat