Will salt water affect credit and other ID cards? (sport)

[ndcairngorm]

We want to take public transportation to the beach, take a minimal amount of stuff and go swimming in the sea, then come back on the bus.

We thought to get waist packs or similar to carry a couple of ID cards, a $10 bill, bus change, and the house keys. None of this needs to be waterproof, does it? Something that latched securely around the waist, didn't flop around too much, and closed with a zipper (not velcro because a big wave might open it) was what we were thinking.

Any ideas?