The speed is 200% underwater and the name of the mount is "Yellow Water Pony" - Does that mean there are other water ponies in other colors? Who knows.
I have reason to believe there are an epic type of mount of this as well.. As Yellow Water Pony is listed as a "blue" type of mount, instead of epic.
Edit: There are other colored water ponies as well, i saw a red water pony but went too fast for me to get a decent picture.