I never used a magnet for this purpose (most of the time the water was merely ankle to knee high) but I would find all kinds of stuff in a small creek at my local park. Screwdrivers, pocketknives, keys, tools, coins, bottle openers, the list went on and on. It was like everyone's tools and spare change ran away from home and jumped in the creek to go swim. Some were bad beyond repair but others I could get with a scour pad and some 3 in 1 oil and they'd be almost as good as new.

Whenever you get the chance to "magnet fish" give us an update and tell us how it went. Good luck, have fun, and stay safe!