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 use the other two maps to flip the direction so that you end up going to the side of the map you want to be on. You do this by plotting a course to one, which puts you on the opposite side, turning around immediately, plotting course for the second map, then turning around and plotting course for the destination map.