If you chose Latios, you will find Latias here and vice versa. Southern Island is where you get the opposite bird.

To get into this city you must dive underwater.

The seven gyms are the Battle Tube, Battle Dome, Battle Factory, Battle Arena, Battle Palace, Battle Pyramid, and Battle Tower. Additionally, there are seven gyms in the Battle Frontier. The Battle Frontier is another region with fourteen more badges to collect. After you defeat them, you will get a ticket from your father to get to the Battle Frontier.

To get to the Battle Frontier, you must defeat the Elite Four with Pokemon only with your I.D.