This is one of those ‘nice to know’ things. You’d be seven types of crazy to willingly store your ammo underwater for a long period of time, but any container that would (apparently) let you do that will certainly do a bang-up job of keeping your ammo dry when its sitting in the back of your truck as you drive through the night to your alpha site, or leave it sitting hidden under some forest debris for a while.
I would be extremely interested to see this sort of test performed using the Chinese knockoff ammo cans. Maybe they’d hold up, maybe not. But on this admittedly statistically small sample, it appears that good condition US ammo cans with good gaskets are capable of doing some amazing work keeping your ammo dry.
As an aside, I’m a belt-and-suspenders kinda guy…if it absolutely needs to be waterproof, put it int a water tight container…then put that in another waterproof container. And maybe vacuum seal the goodies inside in a nice thick plastic pouch.