Simple, suicides or drownings.

People die in the water wearing running shoes.

Then most of the body decomposes, but the feet stay in the shoes, which float on the surface due to the buoyant cushy soles (often containing air bladder cushions).

Sure - that's a reasonable hypothesis, but as the Wikipedia article notes, there are other potential explanations. The level of rarity section speaks to the decomposition process as well as the likelihood of finding feet without other body parts.

you'd think with the new dna 'matching random dna leads to that person's relatives' that keeps happening would lead us to identify some of those remains.