In order to get the mount, you need to fish up 100 x Drowned Mana, and be "Best Friend" with the fishing vendor Conjurer Margoss. Getting reputation with him is also done by fishing up Drowned Mana, and in total you need 935 Drowned Manas (835 for the rep + 100 for the mount). The mount is also sold by Conjurer Margoss, the same NPC you need rep with.
Note that you can only carry 100 Drowned Mana at a time, so trade them in for rep once in a while.
Getting to Conjurer MargossThere are a few ways to reach Margoss' Retreat and fish for the Drowned Mana, and the first method is a bit annoying. It requires an item called Emblem of Margoss, which can be fished up in the Underbelly in Dalaran. This is a one-time use item, so while it's useful, you need a lot of them to consistently get there. Another method is to just get there manually. Go to between the Violet Citadel and the Magus Commerce Exchange and you will see the island in the distance with the lake in the middle. Most classes must use a Goblin Glider Kit to reach it, but Demon Hunters can use the Glide spell.
You can get back to Dalaran by talking to Conjurer Margoss for a free teleport.
Farming Drowned ManaDrowned Mana is obtained by fishing in the pool on Margoss' Retreat, which is right next to the vendor. Each cast will get you between 0 and 3 Drowned Manas, and fishing skill does not affect the drop chance. It will take a pretty long time to get all the Drowned Manas you need for the mount, but there is a trick for faster farming below. However, if you just want to chill back and fish, just go to the correct spot and put on a movie on the other monitor or something like that.
Farming Drowned Mana as fast as possible!If you fish up an item called Mark of Aquaos you can use this to spawn a raid boss. Upon defeating this boss the water will sparkle, and for the next three minutes the drop chance of Drowned Mana will be dramatically increased, with a guaranteed 1-3 Drowned Manas each cast! This is not extremely useful in itself, but if you get a full 40 man raid this boss can pretty much be spawned all the time, so you can fish with a permanent buff, making it extremely much faster!
And another extra tip for faster farming:
- Use the WHEE! buff from riding the carousel at the Darkmoon Faire before turning in motes for extra rep! However note that you can only carry 100 Drowned Mana at a time, so unfortunately you need to reapply the buff every once in a while.