Not saying I know anything about it, but I understand that a pint of black powder and some rocks (for weight) in a quart jar, waterproof fuse stuck through the lid and sealed with a dab of wax, can make a stunning underwater explosion when dropped off the stern of a speeding motorboat.

It might also be possible to mix certain gas-producing chemicals in a well-sealed quart jar and cause it to explode underwater. I don't know this for sure, of course, and you'll have to figure it out for yourself like I did.