Yeah, but it's way harder under water.

I punched a window and broke it back in the day, but that was a junk car from the 80s we were destroying. And I was outside of it with all the room in the world.

Trying to do it while seated inside would have been impossible, and underwater all that pressure against the other side spread out equally makes it really umpossible. It's basically a giant cushion that absorbs and distributes the force. If you do break it, all that water pressure is going to push it straight in your face, and chances are you're just going to let the water in, but not create a whole big enough to climb thru. Certainly not u til your car is full of water and pressure equalizes.

If you're worried about this enough to carry a glass breaker, take a page from the Kia boys and make sure it's ceramic. Even steel with a point is going to be difficult. But ceramic will shatter it with almost no effort. Gotta keep on bipping