well I've just got back from the swimming hole and discovered that
A) I dont sink with my dry suit completely open to the waist
b) due to the pressure of the water against my legs it takes at least a couple of minute for the suit to fill
c) my beaver arctic undergarment also tales a few minutes to saturate
next test is to fall head first with suit open to see if I stay upside down- but i want someone with me for that
Very satisfying.