Un'Goro has 45+ fish but you need fishing skill of 250+ and the forgotten coast of Feralas is also a place to get good fishing , also try Wintersprings and other "high lvl" zones
The highest level area to fish in that I’m aware of is the Sea of Storms. You’ll have to have over 320 fishing to even cast your line. If you want to catch more than 1 out of 4 fish you’ll need a fishing skill higher than 370. Fishing lures are your friends.