That would take FOREVER, even in 100ft water.
The idea is to let the ocean do most of the work. For example, one of the big problems with protected harbors is that they quickly collect silt. But oddly enough, the ancient Greeks built such a harbor that was self-cleaning.
A decade or so ago, someone discovered that an underwater cable with a low current going through it is very attractive to reef building corals, which are drawn to it and encase it in coral much faster than normal. Right now, there is an experiment off the coast of Africa with a cable grid attempting to rapidly create an artificial reef using this phenomenon.
Many other tricks can be used, to produce and drop bulk material below the coral depth. Granted, it will take decades, but once the process is self sustaining, who cares?