Johnnie,
Can you explain your logic?
Why are the barrels hoisted? Can't they be chucked off the side of a boat - maybe with some kind of sea-parachute to slow their descent?
Will the iron from the barrels end up in the food chain?
How long will the barrels last? Surely if the barrels degrade and get taken up by the food chain, they will end up disintegrating rapidly?
What's the significance of the sand/lime filler? Is it there for ballast, or ecological reasons? What's the carbon impact of the lime?
How will this replace reefs? Surely there will be no coral and thus many species will not be able to use the new artificial habitat?