Your two points snap to the nearest nodes of the global shipping-lane network (the open Eurostat maritime network used in freight routing). The engine finds the shortest path through that network, honoring the passages you allow: unticked canals and straits are left open, ticked ones are routed around, and canals too shallow for your vessel draft are blocked automatically. Your exact start and end points are stitched onto the path, the line is simplified to a short list of draggable waypoints, and distance is measured along the path with time from your cruising speed. It optimizes distance over known lanes; it does not see weather or nearshore depth, which is why you refine the waypoints and check the forecast afterwards.