Those ziplock baggies might keep off a light spray of water, but they won't survive a dunking. They also rip and develop holes at the drop of a hat. Putting something heavy like ammo in them will just cause more abrasive wear and punctures. It won't hurt if you use them, but it won't help much either.

Get some real ammo boxes, even cheap ones, and add some desicant in there if you're in a humid area.