don’t try this at home, kids.
If you fire a hot load underwater, the gun could explode in your hands. Why? Increase backpressure due to water in the barrel!
Don’t be dumb.
Glocks are built to handle it. Some require installing “underwater spring cups”, an official Glock product, but those just help water flow for cycling the action.
FMJ only, though. Hollow points WILL cause devestating backpressures.