You use a movement point every time you cross a border between two zones, whether they be sea or island zones. When you leave an island, you cross the border into the sea zone and use one movement point. When you enter an island, you cross the border from the sea zone and use one movement point. When you move from one sea zone to another, you use one movement point. Conversely, if you don’t cross a border between zones, you don’t use a movement point.
Does this help?