70 15 inbetween small island and mainland... keep going south past the small islands there is a river all along there keep following river to mainland going into south scar.
If you go north of 70 15 in ocean past big island across from swamp
Viking Bay cave has high quantities but the sea scorpions are too much of a pain. I go diving in the shallows on the north side at night. Harvested thousands there. Keep en eye out for the odd manta around but you shouldn't have to worry too much.
Like Josh Williams, said. It's the best spot. You don't even need scuba. Beware of eventual golems in the water tho.
I farmed pearls there with a raft. Was low lv (20 ish)
between the grand canyon and the western swamp there are plenty of pearls however the shallow areas are crawling with Sarcos, Kapros, Barryonyxs...etc... however they almost never go down into the deeper areas