Yes, like all fully-skirted hovercrafts she has buoyancy tanks under the main deck.

So if the cushion deflated for some reason, would it still float?

That's not correct in my opinion,BobS,she was definitely more quiet in comparison to other turboprop aircraft. Compare her sound to the deafening noise of the Lockheed Hercules or the ear-shattering noise of the Vickers Viscount, both driven by turboprop engines. The Britannia didn't get her nickname without reason.....

Best regards Hans

The Britannia was "whispering" only in relation to the piston-engined aircraft of the time.

However the engines of the Bristol Britannia produced a very smooth sound.... I remember seeing these wonderful aircraft of RAF Transport Command many times and she was aptly called the "The Whispering Giant".

Best regards Hans

The engines were Bristol Proteus turboprops, as in the Bristol Britannia airliner

That's pretty understandable, Bob. The only "ship" here is that they could float! Their pretty compact wheelhouse reminds like how it's on planes, their transmissions between gas turbines via gears & shafts to fans are close to how it's on helicopters. The whole hull construction is more aircraft-like. Also affected by wind, piloting them could be compared to an aircraft as well...
Interesting how back in the 60s in order to make a high-speed ferry the shipbuilders had to jump in to these air cushion exotics, just to realize in few decades the more wise way is to use multihulls!

Denis: At one time these surface effect craft were deemed not to be ships but aircraft!

Probably wouldn't be allowed to run these days, "excessive noise pollution"!!

Wow, excellent photo, Ken! Magnificent ships those were, but sadly, this is the dark past now.