From the info section: Why do ships sometimes appear to move across land? In some cases this is because there are ships navigating via canals or rivers that ar

From the info section: Why do ships sometimes appear to move across land? In some cases this is because there are ships navigating via canals or rivers that aren’t visible on the map. Generally, though, this effect is an artefact of animating a ship between two recorded positions with missing data between, especially when the positions are separated by a narrow strip of land. We may develop the map to remove this effect in the future.