I have collected a bunch of those ultra-strong magnets from old hard drives (yes, there are ultra-strong magnets in hard drives) and attached them to a hunk of aluminum, which in turn I've attached to a long nylon rope. It's very compact and very powerful.
On dry land, this thing can lift a table saw off the ground, in water it can do even better.
Well, my son an I will be taking this thing to the boat ramp on the Delaware river this weekend and tossing it in and dragging it back to shore. I know I have at least 1 set of keys in there, and who knows what else.