Phaedrus,

Nearly all glass breaking tools are usless underwater. There is no friction with the ground to swing one quickly and water resistance slows it down. Automatic center punches would work but unless there is something to hang onto you cannot push it in to cock the spring.

As to the ear damage, a contact shot will not pose too much threat to the diver as the gas pressure all goes inside. In some areas a Glock is the standard shark defense tool if you are spearfishing. A non contact shot would be loud enough to be uncomfortable but, as has been stated, the alternative is worse. There is the possibility of ear damage but it is small.

Respectfully,

Jerry