Aqua Jewel
Drops from Sahn Tidehunter. Epics come and go, but swim speed is forever until Blizzard detects players having fun with old items.
The 
Aqua Jewel is a Mists of Pandaria toy that drops from Sahn Tidehunter, a rare elite found in the Vale of Eternal Blossoms.
Guide awaiting review
Locate Sahn Tidehunter
Travel to the Vale of Eternal Blossoms and find Sahn Tidehunter at 68.8, 31.6. This is a rare elite, so it will not always be present when you arrive.
Defeat and loot Sahn Tidehunter
Defeat Sahn Tidehunter at 68.8, 31.6 in the Vale of Eternal Blossoms and loot the Aqua Jewel from the corpse. As a rare elite in old content, you can take it on solo at max level.
Use the Aqua Jewel
Using the Aqua Jewel grants you increased swim speed, water breathing, and a translucent watery visual effect that also extends to your mount. The buff persists through mounting and taking damage in combat.
Tips
Shadow Priests should disable Shadowform before using the Aqua Jewel, then reactivate it afterward to achieve a fully invisible appearance rather than just translucent.
