I have used all 20 yet I still did not get this achievement.  Do I have to start and finish a season with the fisherman to get this achievement.  I picked up the last guy I needed part way thru a season.  Also does it only work for King or does it work with Opie season too? 

Just start a season (king or opies) and go around to all the ports and hire all the people you need. At some point the achievement will pop up. That is how I did it.

I finally got this achievement.  I did go and get people back and forth, but I had to fire the last guy I hired and then re-hire him and that got me the achievement.