Just go to eldrich zone on crystal isles and look for big bubbles that have some black clams/oysters inside of them, put your angler fish inside the bubble, harvest, and enjoy the best black pearls harvester in the entire game, super easy, rank up for people to see this.
On crystal isles go to the elderich isles and youll c some giant watery orbs. U can actually go inside them and swim around. Youll c some black clam/oyster lookin things. If u pick those up like u would to berries from a bush youll get black pearls. Just be careful as eels and other annoying things can spawn in the orbs.