I’m not sure about the math, but you can probably buy a lot of concrete with USD $20,000.00. I’d move into the business of creating artificial coral reefs as tourist attractions. I’d probably go further than this and move for the creation of underwater art. Like this.
Get a celebrity altruist-artist involved. You can build these artificial reefs in sandy bottoms where no corals exist. Nothing to lose, really. If coral life starts, then all is well and good. If not, then you still have a great tourist attraction that can spur the SCUBA industry further. Get the diving community involved to help seed these underwater. They’d be more than willing to help. At 40ft depth with relatively strong currents, you can probably mix these designs with giant clams to help accelerate the growth of the reef as a whole. Get the locals to protect the area. Educate. It’s pretty much like establishing more protected marine zones but you have more control over the results because the stuff you seed is man-made. It goes further than just dumping concrete into the water.
To make the project sustainable, get corporations to sponsor reefs of their own. Imagine an artificial reef sculpted to look like a Coca-Cola vendo machine with tables and chairs and people — all made of concrete. The selling proposition would be simple: not only do you directly help the environment, you also create new and enduring tourism properties with your corporate branding.