Top half for storing duffle bags bottom half for storing gear. Paid $25 a crate for 4 of these $8 elastic tarp thing and some eyelets I stole from work. Can store two duffles in the top and the crates split right down the middle so the bottom half can store everything else.
Yes the tongue and groove interlock and then on the outside are latches. We used very similar containers while I was in the Army (this one is also surplus) they are not cheaply made and will take a hell of a beating
They are actually air and water tight with a pressure release valve when closed