I've so enjoyed the "Strangest Thing You've Ever Caught" thread that I thought I'd start a related one--The Strangest Thing You've Ever Seen While Fishing." I'll begin with two offerings--one magical and the second macabre.
For the magical, I look back to a time, right at daylight, when I watched a mother mink ferry her kits (three of them) across the Nantahala River one by one. She carried them just like a cat carries kittens, swimming with her head high.
On the macabre side of things, years ago while fishing in Virginia's Smith River I spotted an oblong object in fairly deep water. As I got closer I realized it was a coffin. Presumably there had been some flooding which had undercut a cemetery somewhere upstream. I reported it to local authorities and never heard anything more.