When this achievement says fishermen they are referring to the crewmen and not the captains as I had originally thought. There are 20 fishermen in all and you can use five per season. At the end of your fifth season you should get it.
What I did was at the start of the first season I picked five of them and wrote down which ones I had selected. Next, play the season and skip the Opilio season. Then at the start of the next king season I would hire five new fishermen. Repeat this process until you have used them all.
Here is a list of the fishermen.....
15. Jake Harris
20. Jake Anderson