Okay. It's actually no so far from the truth anyway. Underwater volcanic activity concentrate gold and other minerals into mine-able quantities, essentially laying out like a giant layer cake. That's why gold runs in veins, and you have to follow the vein to mine it.
Now. You will need to develop technology to not only handle the pressures of underwater mining, but also underwater mining near volcanic activity. Then you have to make it practical.
How much is it worth the gold you are going to get versus the cost of developing and implementing such technology? (Of course, the same thing could be said about an asteroid of gold too).
Somehow, I don't think that's a profitable venture.