I tried the T-bag from tribord design ( http://www.tribordesign.ca/projet4.php )
setup was :
Alexa
cooke s4 40mm
arri wireless remote ( iris )
cam wave HD sender
A.B. 160 batt
T-Bag w flat dome
nd 6
we did some shots on a water park.. haven't been under 3'.. bag is certified to 10' but the designer told me he went to 40' with it.
I would definitly run more test without the camera on bag to go deeper than 5 feet..
if you can, use the hydroflex.