I see how it can look sort of fun. But when you think about it, you are standing at a lake throwing a line in the water hoping to pull something out. Before you see what it is you caught, you have to wipe off all the stinky mud and slime from the bottom of the lake. And instead of fish, you are pulling out an old rusted up piece of scrap. So now you have to decide what to do with this pile of slimy smelly scrap metal... throw it in the trash can, or throw it back in the lake? Or maybe just leave it right there on the bank. Or you can take some reverent pictures and shoot a video for youtube while feigning your excitement and claiming some value in pulling this junk out of the water.
All in all, it sounds a lot like carp fishing to me.
I would think if you throw any trash back in, ( even if you pulled it out ), you are guilty of littering and deserve a big fine.