That mount was created solely and exclusively as a cheap (read: no cost other than doing some quests) method of obtaining an aquatic mount to use in Vashj'ir. No more, no less, hence why it wasn't changed to be usable in all water zones. I'm sorry for saying what you didn't want to read, but if you want aquatic mounts that work everywhere, you'll have to get Poseidus or a Sea Turtle. This is coming from someone who has neither of these mounts, nor the jewel from that rare in Vale of Eternal Blossoms, and absolutely loathes any underwater objectives that don't give a speed buff. I wouldn't complain if the mount was changed, but it's unlikely to happen.