I think you are absolutely right and its limber tail also called cold tail, dead tail, broken wag etc.
You really shouldn't worry about it if it isn't causing her pain. I think the fun she gets from the pool is worth it. Dogs can also get this from just having a happy day where they wag their tail a lot.
You can read up on this here
http://www.woodhavenlabs.com/coldtail.html
http://www.working-retriever.com/library/cwtail.html
If she does seem sore you can see your vet about some pain meds but otherwise I'd just put it down to part of her being a happy lab.
Hope this helps you!
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