The Great Sea Ray is an aquatic mount that can be used underwater, and will severely increase the travel speed under water compared to swimming. In order to get this you will need to go finish in any Battle for Azeroth zone in either Kul Tiras or Zandalar. It is pretty much a random fishing drop from any catch in these zones, but at a very low drop chance.
Because if the random nature of the drop, there is no good strategy for farming this mount beside just fishing for as long as you can manage. Some users have reported getting it in a few minutes, while others report having fished for over 50 hours before seeing it drop. It's best to be patient and perhaps even have this as a secondary objective when fishing, or else you might just go insane from all the fishing you need to do to get this.
The mount used to have a much higher drop chance around the launch of Battle for Azeroth, but it was heavily nerfed after a few weeks. This is the reason why you will see a lot of users reporting several mounts per hour from farming this. But just keep in mind that this is not really the case any longer.
The Great Sea Ray is Bind of Equip, so you can also buy this mount of the Auction House if you don't like fishing. Likewise you can sell it if you prefer to get some gold instead of another mount.