A good example of wreckage recovery from the bottom of the ocean was the Air India 747 that was lost off the coast of Ireland. A British vessel located the wreck site by following acoustic pings from the recorders, and a French cable-laying vessel used its remote submarine to recover both the FDR and CVR from a depth of about 7,000 feet. This all happened over a span of about two weeks.
The pingers can be heard for something like 2 miles, and will last for 30 days.
Later, the Canadian investigators (the flight originated in Montreal) recovered much of the wreckage as part of their accident investigation.
The average depth of the Atlantic ocean is something like 12,000 feet.