I honestly had no idea until today that little island underneath Dalaran is the fishing grind for Legion. And just like last time, there was a 25 man raid doing it through LFG. It was 6 solid hours of the following process: fish up the Mark of Aquaos, summon the water elemental, all of the ranged blows him to pieces, and fish under a rain of mana for about 3 minutes. Turn in if you get to 100 Drowned Mana.
Gets you an underwater shark mount, a tradeable Sting Ray Pup pet, a non-tradeable Trashy raccoon pet, and two toys that give you different bobbers - a murloc head and a rubber ducky.