I'm not sure if this is Mac-specific, but several times since the recent patch my characters have been stuck in the swimming animation after dismounting from their flying mounts. The animation persists even after I subsequently re-mount then dismout. The only way I've been able to stop it is by relogging.
It seems that having the Broom in your inventory will cause this. If you're still carrying that around, go ahead and remove it.
Thanks for the replies. None of my characters have a broom in their inventory. My latency is typically <300 ms.
I'm getting this as well, no broomstick, latency typically<200 (though I guess there could be latency spikes that cause it). It was kind of funny at first, but starts to look weird after a while.