The Inkscale Deepseeker is a new mount added in patch 8.2 of World of Warcraft, and this mount is tied to PvP activities in Nazjatar. If you want it, be prepared to participate in the World PvP events that take place all over Nazjatar, and you obviously have to turn on War Mode for this. Basically a new event takes place every 45 minutes, and both factions have to fight to gain control over 5 different control points (located at Tidal Conflux, Alathi Spire, Thoras'tal Spire, Zin-Azshari, and Lemor'athra Spire).
Whenever you control a control point, your faction gains a point, and the faction with most points after 45 minutes (or if they reaches 1,000 points) win. The winning side get 5 x Nazjatar Battle Commendation, and the losing side only get one. You can also get the commendations from certain quests and world content.
This is important, because the Inkscale Deepseeker mount can be purchased for 150 Nazjatar Battle Commendation.
The best case scenario is that you will need to win the PvP event 30 times to get all 150 Nazjatar Battle Commendations, or roughly around 50 times if you got a 50 % winrate. Some battles tend to be over quick, so while it will take a lot of wins, you might not have to spend a very long time grinding these. You can also participate a bit, then run and do some world quests and still be rewarded with Commendations, as long as you stay in the zone.
When you have 150 Nazjatar Battle Commendation, go to Crafticus Mindbender (Alliance) or Dazzerian (Horde) to use this currency item to buy the Inkscale Deepseeker mount.