Bullets don't travel well in water. They slow down, veer off course, don't hit the target. Guns don't behave well under water either, even if you do find a way to keep your powder dry, there are still other problems to overcome.
But it's not that Glocks can be modified for use underwater that's scary, after all, modifying them for the purpose reduces their killing power. What's needed is a whole new weapon, one that shoots liquid bullets, and the military is busy working on it. Lucky us.
If you're geek enough, you can read the actual patent here.