Umbral Isles has the best fishing spots in the game from a money-making standpoint, a high-level fishing standpoint, a record-breaking standpoint, and from the sheer cool-factor of the fish that can be caught there, so yeah, for a fishing enthusiast it's a must-have place to unlock. Not sure why it's like the only field location in the game that needs x amount of story completion to access, though. Drove me nuts for the longest time.