Help Me Solve the Mystery of the Floating Onions

I took a drive with my oldest daughter tonight to spend some special one-on-one time. (She revealed to me today that she didn’t want baby sister anymore, and I should put her out in the flower garden.) We drove several miles away to a low-water bridge, and poked around along the water. While there, I encountered the strangest thing: a crop of floating onions. They were growing on either side of the bridge, and appeared to actually be growing in the water, rather than having been thrown out there. I’m intrigued by this anomaly, and wonder if they’re common. I would love to hear from you if you have any ideas.

This is her "cheese" face.

Strange "floating" onions.

These were almost fully submerged.

This was the first one I noticed, which was the most "earth-bound" one. I picked it up and it was soggy.