Black Pearl - Novigrad/Skellige
Area: Novigrad - Skellige
Quest Giver: Nidas
Other Requirements: The Destination: Skellige Story Quest will need to be completed to finish this quest
This quest begins in Novigrad by speaking with Nidas outside of The Golden Sturgeon Inn. I'd recommend picking this quest up as soon as you see it in Novigrad as it'll become unavailable at an unspecified later time and you'll be unable to complete it.
Nidas will ask you to meet him at a location in Skellige which is found north of Arinbjorn or south of the Eldberg Lighthouse, if you have that location marked on your map. It's actually quicker to fast travel to the Lighthouse then head south than it is coming here from Arinbjorn. Either way, you'll find Nidas on a broken bridge (pictured below) at the quest location.
On the shore Nidas will fill you in on what you have to do - go swimming and find him a Black Pearl. Your quest will update and you'll have to swim out to the quest area on your map and investigate the clams here for a Black Pearl. You'll find the Black Pearl in a clam on the far end of the quest marker - as far away from the shore as you can be.
After collecting the Black Pearl return to the shoreline where you'll find a group of three Drowners fighting Nidas. You'll need to protect him, letting him die will fail the quest. During the dialogue after saving Nidas he'll tell you that he didn't bring the coin with him, you'll have to meet him back in Novigrad to get your payment. No biggie, once in Novigrad meet Nidas at The Golden Sturgeon and accept your payment; the quest will complete here.