you obtain a sea horse after some quests trieng to ride a naga, a turtle and stuff. theey might add more underwater mounts, note you get a buff when you are on the waterfloor your movement speed while walking is increased by 60% and you can ground moutn and run with 100% on the floor.
When I tryed mounting today in Vashir, it didnt worked, and the Sea Turtle also didnt worked.
Instead I saw alot of people riding sea horses.
So I think from now on, you only can use specific water mounts in Vashir.
But I hope blizz makes the Turtle a water mount too, since it kinda fits in the setting.