I've read it takes 93 Athena chests to hit level 10. That could equate to a modest 232 hours of voyage to reach, which would probably be a large chunk of the journey from sailor to pirate legend (it took me 75 days to reach legend with maybe 5 hours gameplay per day with a week off).
To help ease this grind, I propose the final Athena dig map have the chance of between 1-3 chests to dig on the island, with a favoured weighted chance of between 1-2 chests.
This will help speed up the process without making it a walk in the park.