To unlock this achievement you’ll need to rank up one of your own ships, so make sure you own one. (A sloop can be purchased in the main menu for 250k gold)
When I say “rank up”, I mean you need to get one category of your ship milestones - “The Gold-Seeker”, “The Voyager”, etc - up to 50. There are several categories, and several ways of ranking up each category, but I believe the method I used is the easiest, most foolproof way:
Just sleep onboard your ship. …no, it’s not exciting or particularly heroic, but if you’re a pirate of modest ability like myself, this is the way to do it.
Sleeping on board your ship for 5 minutes (cumulatively) earns you 1 rank of “The Rogue”. Unlike most things that rank up your ship milestones, this can be done almost entirely passively, and takes much less time than the other passive methods, like sitting aboard your ship or playing shanties.
At 5 minutes per rank and 50 ranks needed for the achievement, you’re looking at 4 hours and 10 minutes of catching ZZZZs on the high seas. Unfortunately, your pirate will only sleep for 5 minutes at a time, but there’s nothing stopping you from immediately going back to bed.
You could knock this out in one lengthy session, or you could do what I did and get a few minutes of shuteye before logging off for the day and do this is segments.
Note that you will also rank up your Pirate Milestones this way, getting you to The Rogue rank 50, and unlocking the Professional Pirate achievement as well. It’s not glamorous - real pirating rarely is - but it’s a foolproof way to get two fairly rare achievements with very little trouble. Worse case, you get your ship sunk, respawn elsewhere, and get back to sleep with only a few minutes lost.