Vertical and horizontal shoals
Shoals are the most universally traversed terrain in AWBW. Only Piperunners and non-Lander or Black Boat Naval units are prohibited from traveling on Shoals. They are also easy to travel across, regardless of weather; with the exception of Air units, Shoals always cost only 1 movement point to traverse. The downside of this is the fact that Shoals provide no defensive bonuses whatsoever, so units left on Shoals will be vulnerable.
Shoals, alongside Ports, are the only tiles that allow Landers and Black Boats to Load and Unload units. To unload units on land, the Lander or Black Boat itself must be on the Shoals tile, they may not unload units if they are on a Sea tile.