Swimmer's tail in dogs happens when the water they swim in is either too warm or too cold or they've been swimming for a long time and aren't properly conditioned for the activity.
Juno got swimmer's tail.
We don't know if it was because the water in our pool was, in fact, too cold for her at the beginning of the season (our first year with a pool), or because she got too excited and swam too darn hard for her first time in. She loves swimming, so she knows how to do it, and we know how to read her signals for when she's getting too tired, but she got it anyway.
Luckily, Juno's swimmer's tail only lasted 3 days and didn't seem to bother her too much - we could still touch and lift her tail, and she wasn't in any visible pain. She did chase her tail more than usual, and was a bit lethargic, but for our dog, lethargy is a relative state - she still had a lot of energy for a dog, just maybe a little less than 'her normal'.
But the tell-tale signs were there; a tail that shoots straight out about 3 inches, then hangs limp to the base, even when she's happy or excited. It was sad - and we felt so badly!
Juno's tail hasn't been plagued by swimmer's tail since - our pool water has warmed up and she's building up something of a swimmer's physique (!), so she can now easily out-swim her previously un-swim-conditioned self from earlier in the season. Here she is in better form:
She has to wait for her 'swim word' first...
But check it out, for any dog's sake. You can read more (it's also referred to as limber tail syndrome, broken wag, etc.) here - http://en.wikipedia.org/wiki/Limber_tail_syndrome
and you can see it in action below - (we didn't have the heart to capture Juno's particular experience on camera or on video):