Hey Vince,
A little duct tape (three layers) will go a long way toward sealing that metal box, unless you're talking about leaving it underwater for weeks at a time. The up side is, you can use the duct tape for other uses as needed, and the metal box/duct tape set-up's a WHOLE lot cheaper than a TRULY watertight plastic box. If you're looking for an underwater cache, go with heavy walled PVC, end caps, and silicone tape.

Troy