Vashj'ir is a fun zone to level in for Classic's Cataclysm expansion, except for one thing—it's entirely underwater, making go rummy you both slow and prone to drowning. Fortunately, an early questline provides you with underwater breathing, the ability to run quickly on the sea floor and a Vashj'ir seahorse mount, faster than any other for travel underwater.
Not only will the holy rummy Abyssal Seahorse mount be handy for getting around while questing in the underwater zone, but it will make traveling to the Throne of the Tides dungeon much easier. If you're a hunter, it will also speed you to the area where the rare is found. With all that in mind, here's how to get the Cataclysm seahorse mount for speedy travel.
How 
to breathe underwater in Vashj'ir
You have choices for where you want to level as you start Cataclysm. In this case, you want Vashj'ir, which you can get a breadcrumb quest for from any Warchief's Command Board in Orgrimmar for Horde players or Hero's Call Board in Stormwind for Alliance. Follow the quests and they will take you on a ship to the underwater zone, in an adventure that… doesn't end well.
You'll awaken in an underwater boat that has cleverly trapped some air, next to Erunak Stonespeaker, a Draenei that begins the Vash'jir questline. He'll give you a quest called Sea royally rummy Legs, which will grant you the ability to breathe underwater and run 60% faster while in the zone. There's a reagent vendor and an anvil that acts as a vendor to repair your gear nearby.
To complete the quest, you'll loot three starfish and a conch shell from the sandy bottom near the boat. Purple clamshells with bubbles will refresh your breath, so you won't drown. Once you hand it in you can breathe and run underwater.
How to get your seahorse
It's about 15 minutes of questing to get your seahorse from the moment you arrive in Vashj'ir if you're efficient about it.
Progress through the storyline, collecting food and equipment, clearing out naga snake creatures and unfriendly aquatic goblin-like gilblin, and become generally amazed by the pretty scenery and the massive aggro ranges of every unfriendly mob in the area.
Keep up to date with the most important stories and the best deals, as picked by the PC Gamer team.
You'll attempt to defend your ship through an onslaught, and will eventually be taken to a small underwater camp for a few more quests, where you'll attempt to ride a sea turtle which won't go so well. But then NPC Moanah Stormhoof will have another idea—you should lure and subdue an Abyssal Seahorse, in a quest called The Abyssal Ride.
Tie off a cute little fish on a lure, and the seahorse will wander along. Hop aboard and you're in for a little rhythm game, consisting of pressing buttons in the correct order, to subdue it and make it your own.
Complete that quest and you'll be given the seahorse mount as an item in your bags. Right-click the item to learn it, find it at the top of your mount collection (since it helpfully starts with A), and you can now ride around anywhere underwater—and only underwater—at superspeed. It increases your swim speed by 450% so enjoy your zippy swimming!