If they are Military Ammo Cans, and closed properly, they should be water-tight to well over 100 feet. 'This is a good guess, and not a deeply researched fact, but as I deal with these cans on a regular basis (USAF here), I would say, a very good guess at a minimum.'
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I currently have 2 ammo cans filled with bricks laying in the 10 foot end on the bottom of my pool. I check them every now and again. Bone dry. I've been letting the neighbor kids pull them around the bottom, and changing depths to see if pressure is an issue. none have arisen so far.