I think you would be spending dollars to collect dimes.
To draw on a reality show, there are folks who dive-mine the Bering sea and near-shores of Alaska and find a good amount of gold with relatively simple equipment. I’m not sure if anyone has ever bothered to look at California off-shore since it gets pretty deep pretty quick. There’s an equivalent to the Grand Canyon out there, and we still pull gold out of dry desert placer claims in Arizona, so maybe there’s the equivalent to the gold-bearing hills of California or sands of Alaska, out there, too, just waiting for a deep rig to suck it up? It might be spending dollars for pennies when it does come to the gold extracted but the main purpose of the plant would be water, so any pennies would be a bonus.