If you plan on carrying SPOT on a hydropack , consider a padded zip pouch designed for vertical strap mounting like this one from Aerostitch:
The P/N 9100 pouch (4.5 in x 2.7 in x 1.6 in) works great, either with or without the plastic SPOT belt clip in place. Removing the clip makes the package a little thinner, but would make it impossible to remove and clip SPOT onto your belt.
I plan to use this pouch exclusively when motorcycling, since I am concerned about whacking SPOT with a branch and ending my ride only to discover that it's lying on the trail 20 miles back beeping at the sky.
It also hides the unit from prying fingers and gives additional protection from the elements. With the pouch mounted up high in front of your armpit, the antenna placement is as ideal as you could get without taping it to the top of your helmet.