I'm not sure about shark dispatching, though I suppose it would work. It certainly makes sense that an UW military or paramilitary unit carrying Glocks would consider them a shark repellent.
I took the Glock Armorer course a while back and we discussed the marinization process, which really is just replacement of a couple of small parts. The instructor then went to great lengths to describe for us the litany of cracked bathtubs and pools, and some really nasty injuries that have occurred through people testing their weapons while underestimating the pressure wave generated underwater (including one fellow who cranked a round off in a bucket and darn near lost his hand from the shock wave). Their recommendation for police armorers was to install the parts, but not tell any of the officers their weapons had been converted. Glock will only sell the marine spring cups to armorers with a police department letterhead indicating they are approved by the head of the agency. I suspect there are aftermarket parts available.
My personal thought is being seen carrying a firearm of any kind into or out of the water would be an excellent way to attract the full and undivided enforcement attention of any Coast Guard, wildlife or marine patrol officer, or local deputy.