[Mullins]

For constant weight it wouldn't really matter how buoyant it was, it would just need to be incompressible i.e. contain no gas. I'm not aware of anything like this, but there are hard neoprenes that contain fewer gas bubbles than those normally used for wetsuits. Unfortunately though it's the gas that provides most of the insulation...

Will's Orca was pretty thin but still made of neoprene. Orca are making us 1mm suits for world champs that should compress very little, but of course they won't be very warm.