In order to unlock the Sjarken boat, a few other boats, and this achievement, you have to travel 20 km manually. Fast travel does not count towards this achievement. It should come naturally since you have to remove the "Fog Of War" in order to locate more hotspots for fishing, and to get to other ports.
Going 20 km also unlocks the Sharky, Selfy, Fix and Follabuen boats.