Brinedeep Bottom-Feeder

To access Conjurer Margoss at 44.6, 62.0 in the Dalaran Sewers, fish from Swirling Whorls fishing nodes in the Black Market area of the sewers to obtain an Emblem of Margoss. Use it to teleport to Margoss' Retreat — note that Emblems are consumed on use, so you will need to fish up another one each time you return.

Fish on Margoss' Retreat to collect Drowned Mana. Each Drowned Mana turned in to Conjurer Margoss grants 50 reputation, and a repeatable quest grants 250 reputation. You need to reach Best Friend status (approximately 840 Drowned Mana for reputation) and then an additional 100 Drowned Mana to purchase the mount. Occasionally you will fish up a Mark of Aquaos, which boosts your Drowned Mana gains. Joining a group via the Group Finder on Margoss' Retreat greatly increases the rate at which Marks of Aquaos appear, allowing you to chain them and speed up acquisition significantly.

Once you have reached Best Friend reputation and have 100 Drowned Mana, return to Conjurer Margoss at 44.6, 62.0 in the Dalaran Sewers and purchase the Brinedeep Bottom-Feeder. The mount increases your underwater swim speed to 135% as of the November 8, 2016 hotfix, which is equivalent to normal unmounted run speed on land.