Because there is nothing forcing them to stay in waterways (they are land units, and thus can move anywhere land units can move) - which is the obvious answer, but I am not sure how to keep them ocean bound.
Remember - The beatings will continue until morale improves.
Tbh I dont see the problem. There is hardly a place in Europe without water, and a slow-moving 1914 seaplane could land on any large enough lake and river. In WW2 Russians operated even from frozen water.