Thanks so much for the additional information. I really appreciate it.
Without being able to see your dog personally, it's hard to say what exactly could be going on...however, since she spent a good part of the day in the water yesterday combined with the fact your dog is a hunting dog makes me wonder if perhaps your dog has 'limber tail'...or as it's called in Northern Minnesota, "Lab Tail"
We see many dogs with these flacid, painful tails and the majority of the time, we see it on Labs, although any dog can come down with it. I've seen it mostly in hunting dogs, but I've seen it in Chihuahuas and Bichons too.
You can find more information on the problem here:
In our practice, we generally treat with some kind of anti-inflammatory and rest. The tail generally heals itself within a couple weeks. Since it is a painful issue though, the best thing to do is to have her seen by a vet as soon as possible so they can get her started on some anti-inflammatories ad get her feeling better sooner.
I hope this helps!