Aquatic mounts

Aquatic mounts are mounts used in and underwater to increase swimming speed, and unless amphibious, can't be used like normal mounts (except in Nazjatar with a Budding Deepcoral). Their normal speed is 160% swim speed or 60% faster than regular swimming.

Drops

 * Fabious (mount), drops by using a S.E.L.F.I.E. Camera MkII on Fabious in Nazjatar.
 * Reins of Poseidus, drops from Poseidus in Shimmering Expanse or Abyssal Depths.
 * Surf Jelly, is a possible reward from Island Expeditions.

Vendor

 * Brinedeep Bottom-Feeder is sold by Conjurer Margoss at Margoss's Retreat for.
 * Crimson Tidestallion is rarely sold by Mrrl in Nazjatar, requires extensive, time-consuming preparation beforehand and War Mode activated. Awards an achievement.
 * Darkwater Skate is sold by Galissa Sundew on Darkmoon Island for 500x Darkmoon Daggermaw.
 * Inkscale Deepseeker is sold by Crafticus Mindbender/Dazzerian in Nazjatar for 150 Nazjatar Battle Commendations.
 * Saltwater Seahorse is sold by Captain Klarisa in Tiragarde Sound/Captain Zen'taga in Zuldazar for.

Professions

 * Great Sea Ray (mount) can be fished from the ocean waters surrounding Zandalar and Kul Tiras.
 * Pond Nettle can be fished from the lava of Krokuun and the Antoran Wastes.
 * Sea Turtle (mount) is a very rare catch that may be obtained while fishing in level 70+ zones, Shipwreck Debris around Darkmoon Island, or a rare drop from Lunarfall Cavedweller / Frostdeep Cavedweller with a level 3 Fishing Shack in the garrison. Awards an achievement.

Quest

 * Fathom Dweller is a world quest reward from DANGER: Kosumoth the Hungering.
 * Vashj'ir Seahorse is a quest reward from The Abyssal Ride.

TCG

 * Riding Turtle was originally from a Saltwater Snapjaw "Loot Card" from the Heroes of Azeroth expansion set. Can now be a rare drop from Lunarfall Cavedweller / Frostdeep Cavedweller, and found on the Black Market Auction House.