Stick to the eastern path at first, then the southern path. Once you reach the room at the end, Doga will modify the Nautilus so it can travel underwater, then transport the party back to the World Map. With the Nautilus so modified, the game really opens up. Several optional Areas become available, and you have full access to a wide variety of Jobs to combine as you see fit. Take the time to experiment with different party setups and find out what works for you.