Magnet fishing, also called magnetic fishing, is searching in outdoor waters for ferromagnetic objects available to pull with a strong neodymium magnet.[1] Recovered items may be dangerous, such as firearms, ammunition, and bombs.
I'm not getting the 'better' part. You're much more stationary and whatever you pull up will be coated with muck and slime and, as what I pasted said, it might also seriously hurt or even kill you.
I'd say finding a bomb on a beach that no one has discovered yet is less likely than finding one in a river.
There isn't even one instance of an active bomb exploding... And magnet fishing is a pretty common thing. Plus generally people will call the police if they find something potentially dangerous or used in a crime. It's probably fine.
I remember watching a video where some people pulled a landmine out of a river and the cops were so annoyed to have to deal with it. Apparently this wasn't the first explosive that's come up out of that river and the cops basically blamed the people for pulling out another one.