If you want to travel to another map via boat you need to plot a course to the map you want to go. Right click on the boat and Plot Course to the destination map. Drive the boat to the edge of the map and you will transition to the map.

Your boat will appear on the far side opposite of the direction you were heading. For instance, if you head west you will end up on the east side of the destination map.

You can get around this by first going to the third unrelated map and turning around immediately, plotting your course for the destination map. You will end up on the same you originated from.