The only pearl in the game is the one to open a certain lock.
When you see the cutscene of Namor guarded and trapped in a bubble take out the guards and use the console to free Namor. Now, from here the side door leads to the Temple of Negrete- this has the seaweed needed to cure Namor. Head left and take out the enemies in the room. The clam will have the pearl you are looking for.