Disclaimer: This dangling plot thread probably has or will be addressed, but I own this take on it.
This depressing little plotbunny was spawned by the same repeat watching as Insurance. (shrugs)
(Edit 11/05: as per my new policy, anonymous reviews get answered in my profile.)
Ten years is a very long time to be trapped in absolute blackness.
After he came around from drowning the first time, there wasn't much to do for a while except fall. Eventually there was the long, slow impact with the seafloor. He presumed it was seafloor, anyway, since there hadn't been real light since the first few fathoms from the surface. Not being able to really feel what he was touching was a bit of a handicap there.
It is very hard to measure the passage of time when there is no marker for it.
Things sometimes nibbled at him, and there was once a sort of glowing fish which provided a pleasant diversion for a while, to watch that moving luminous spot until it vanished from his field of sight.
There isn't much to do when you can't really move.
The cannon proved very securely fastened in a position where he couldn't reach it. He eventually concluded that until either the cannon rusted or the rope or his boots decayed, he was stuck. Perhaps his son might still be alive by that time. That would be nice. Maybe he would get a chance to pay back that vile Barbosa. That would be even nicer.
He spent a lot of time thinking down in the darkness, doing his best not to go mad. He wasn't always sure if he succeeded.
And then the day came when his heart started beating again, and he choked again for want of air, and he had barely a bewildered instant to realize what must have happened before the tons of unfeeling black water above him crushed him beyond any hope of recall.