As you all know, the game map is a projection.
Consider the two waypoints placed on the representation above.
Since we are in the northern hemisphere, the trajectory will not be a straight line due to the spherical figure of the earth, and the vessel will change its course as to reach the second waypoint, it really happens, for long ways.
Moreover, like an arrival circle, which get you arrived when you enter an area 20 miles from the buoy final waypoint consists of a zone of 5 miles, which validates your waypoint a when you are inside it at the next server refresh. So your boat may not browse the all 5 miles inside the waypoint area.