I've never had this problem but yesterday I went swimming and when I came back above water, my speed reduction wasn't removed. So, now I am slowed as if I was underwater yet I'm not. The server hasn't reset yet so it might fix itself when that happens...
I just want to add: it seems that all of the speed reduction bugs that I've seen (from water transitions, web spells, grease, spells, etc.) can be debugged via the casting of a freedom of movement on the character.
Ah good to know, sadly I'm not a druid! :(
Did you go back into the water again and come out a second time? That fixed it when it happened to me.
Awesome that worked!!!!