I suddenly feel the need to learn scuba in the Sea of Cortez and give the sea floor a good looking-over.
Haha, a dive in the SoC for any reason is a good one. All kidding aside, apart from a few dubious claims, I'm not aware anyone has ever found a round pearl while diving (blisters, sure).
Mantle pearls burst from the sac quite often. One they find their new resting spot, they tend to become fused to the the shell by new layers of nacre, hence a conjunction, which is double pearl, if you will. I enjoy studying these because they are excellent indicators of the pearl's age relative to the age of the shell.
Apart from blending with it's surroundings, any expelled pearl would become obscured by sediments within a few days, then become encrusted within weeks. Much like the shells from demised mollusks, the protein content of the pearl would be ingested by microbes over the seasons as the aragonite reverts to calcite. Pearls have survived ancient shipwrecks, but generally because they were preserved in an anaerobic situation, as opposed to cast upon the surface of the substrate. Even then, many of these pearls have lost lustre or become stained.
I really doubt attempting to salvage pearls from the substrate of pearl farms would be fruitful. If anything, ones found would likely be mere nuclei, under-mature or otherwise malformed and damaged.
Archaeology or forensic purposes perhaps, but certainly not to produce marketable gems.