Fisherman, sailor, or other unfortunate gets washed overboard. Sea creatures eat the remains but can't get at the foot in the shoe, which also protects the foot from decomposition. Foot detaches from rest of remains, floats with running shoe, drifts to shore. Folks develop conspiracy theories.
So it goes.
All these sea going people just happen to be wearing running shoes, ok sure......
Fisherman - having had the pleasure of falling overboard, I can attest that the first thing one does is get rid of the footwear usually boots or lose fitting slippers - you cannot swim in shoes, boots, or slippers.
Those aren’t the kind of shoes fishermen and sailors wear. I worked at sea and no one would ever think of wearing those.