I got introduced to Magnet Fishing when I went to my local FINS lake (Mike Miller Park in Marshall County) to avail myself of some of the stocked trout. On the fishing pier was a fellow MFing pulling all sorts of stuff from around the pier.
I went to Harbor Freight and bought the biggest magnet they had:
Harbor Freight retrieving-magnet-250-lb-pull
Rule #1 - You want a braided rope, not a twisted one. A twisted rope will try to un-twist if you hang something heavy. In this case bigger is better - to a point.
Rule #2 - You want a bucket to store your rope. Braided ropes take forever to dry, and you don't want that nasty rope in your car or truck.
Rule #3 - You want heavy waterproof gloves. You will drag up some really nasty stuff. Harbor Freight has some inexpensive ones (buy two pair).
Hint #1 - For docks and piers ask the owner if you can MF off the dock. Tell him/her you will remove any old rusted metal under the dock.
Hint #2 - The area adjacent to a bridge over a river or creek belongs to the County or City. If you should try that area, expect to be told by the local constabulary that MFing is not allowed there.
Hint #3 - Check YouTube which has a gazillion videos of MFing.
Request #1 - Bring and use a small digital camera (or use your phone if you trust you won't drop it in the water) and photograph everything you bring up. Showing the dock owner a picture of the 20 feet of barbed-wire makes them very appreciative, and more likely to let you try again.
Share your pictures.