A crew of 20 men with four "canoes" Could get calcium rich rocks from the smaller uninhabited islands. Set them in shallow water on the lee side of the island. Wait for the coral to grow and use a small dredge to pump the coarse sand these islands are made of.
I have a 115hp 4 stroke Yamaha on my fishing boat. Coarse coral sand will not go through the water pump. So if you're hunting oysters or beche, have to get out and scrape the sand off.
