I and HWIAND had the same thought about Turner Sr.

Or, for that matter, since they thought they needed the coin back, why didn't the Black Pearl folks go and get him back and bring him back up?

The only thing I can think of is that there are effective limits to depth or something: they can walk across the bottom of the ocean so long as it's relatively shallow, but maybe they tossed Turner Sr. down a deep crevasse or something.