Requires a Reef (the red/blue item along the coast) and must be built on the coast. Produces Pearls, which are only used by Pearl workshops to create Pearl Necklaces, a property need for Envoys. One Pearl fisher can supply one Pearl workshop.
You can build as many Pearl fishers along a reef as can fit within the area. Generally this is 4 to 6 total Pearl fishers per reef. Consequently, you would build an equal amount of Pearl workshops to match the number of Pearl fishers to optimize production.