it's called limber tail syndrome or cold water tail, not that uncommon. I've seen it before in one of my "long tailed dogs" here's some info on it.

Limber tail syndrome and "cold water tail" while known to those who work with hunting dogs, may not be familiar to veterinarians. It is most often seen in working breeds like English Pointers, English Setters, Foxhounds, Beagles, and Labrador Retrievers. Ages of affected dogs range from 0.5-9 years old. In English Pointers the most frequent age of onset is reported to be 2 years old.

Typically the presentation is a young adult dog with an acutely flaccid tail that hangs down from the tail base or is held horizontally for 3-4 inches and then drops down. The tail remains in this position even when the dog moves about.

Pain may be seen on palpation of the tail base and some owners report that the dog seems uncomfortable and painful. The best thing to do is leave the tail alone.

Rest is recommended. Complete recovery is usually seen by 2 weeks and often occurs within a few days although it recurs later during training in 1/3 of the cases. Some owners and trainers feel that anti-inflammatory drugs shorten the recovery time if given when the condition is first seen. You might also use warm packs at the base of the tail which will help the relief of pain.

The cause of limber tail is not known although it is thought to be associated with hard workouts (especially in underconditioned dogs), heavy hunting, and swimming or bathing in water that is too cold or too warm. Some owners reported that they grab the tail as a means of correction. Tail conformation (high set or very active), gender (males more frequently affected), and nutritional factors have also been suggested as possible causes. Ongoing studies suggest that limber tail is associaed with muscle damage in the tail with dogs examined early in showing elevated muscle enzymes eg., creatine phosphokinase.