Fishing skill doesn't have any effect on how many fish you catch in the new Battle for Azeroth zones. You will catch the same number of fishes with Fishing skill 1, as with Fishing skill 175. The only reason to level Fishing is for Achievements. The new BfA Fishing skill is named differently for the two factions, but the name is the only difference between them. Kul Tiran Fishing is the Alliance version and Zandalari Fishing is the Horde version.