I'm lying here partially-watching/partially-listening to videos by people who "mudlark" (search river mud for items), "fish" with magnets, and metal-detect. It's fascinating what they find!

Do any of you do any of these as a hobby, or know someone who does? What have you/they found?

I don't personally have the hobby, but when I was growing up, my best friend's dad would always take a metal detector out to the islands and beaches around our area.  There are legends of buried pirate treasure all along the barrier island coastlines, and I remember one time he did actually dig out a gold coin that had washed up.  We were too young to really understand the significance of it, but I remember being infatuated with the idea of digging up buried treasure chests filled with jewels and coins lol

I'm not sure what ever happened to it or specifics on other items he would find, but I wouldn't be surprised if that wasn't the only thing he ever found.

I do however love to look for/find shark teeth, but that isn't a common perception of a "treasure" :)

ohhh that's awesome. I live in Minnesota so there wouldn't be any fun to it here haha but now this is something I need to find online to watch at least. xD

That's cool, Vida! I'd say islands would be a pretty good source of items, definately beaches! And the dream of finding pirate treasure is one many people had when kids! lol
And I sure would find shark teeth a great find! :o

Pants: It can be done pretty much anywhere, and all sorts of objects have been found!
I follow a few channels on the topics I listed. This is a nice one: https://www.youtube.com/channel/UC2K7yEwPIcPaQT5FM78dpyw

There's loads of videos on YouTube!

I was always obsessed with watching people use a metal detector when I was a kid, but I've never used one myself.