So if I read this right from page 43:

If, due to it's chosen maneuver, a ship would move so that, at the end of the movement, its base overlaps a reef terrain element, movement is not executed in the normal way.
If I sail entirely across the reef in one move, so that none of my base remains in contact with it at the end of the move, I can cross it with no effect. It's only if I end up with any of my base on or touching the reef marker that I suffer damage and risk grounding...?