Is the meaning here that it's probable that the builders erected the structures under water? 
Is the meaning here that it's probable that the builders erected the structures under water?
Aha! Enter my theory: The Gulf Of Mexico was blocked off (from the world's oceans) through Florida, Cuba and the Yucatan during the Ice Age. Over thousands of years, the Gulf partially dried out and eventually created a stable but reduced water level in the Gulf. The builders of this 'city' built on the shoreline of the 'reduced water level Gulf.'
When the Ice Age began to end and the world's oceans began to rise, the 'dam' broke and re-flooded the Gulf, submerging these structures. (just like the flood that broke through at the Bosporus and flooded the Black Sea, "Noah's Flood.")
Some are saying that The Bimini Road is associated with the civilization that built the underwater 'city' and was all part of a massive complex in that region. (Throw in a few earthquakes, some volcanic activity and maybe even a meteorite.)