The bag isn't full... the top part, down the sides is just a floatation tube similar to one on a RIB. The actual skirt is below that.
All hovercraft should float without the engine(s) running... when afloat and not hovering it becomes a very slow boat. I say very slow becaus the bag or skirt drags in the water and stops the craft getting up to more than one or two knots.
In case you wonder how I know... I have my own hovercraft (2 seater) and I am a member of the Hovercraft Club of GB (HCGB). /forums/images/graemlins/smile.gif