Once you've beaten the Elite Four a lot of things get unlocked for you to do. First, go to the battle tower and beat 7 trainers in a row. You'll get some nifty reward. After that, go to the breeding center west of Muaville. You'll need to trade with a friend so you have both Groudon and Kyogre. Give both of them to the breeding center to raise, it doesn't matter what order. Once they're both in, go back to the battle tower, but take with you a team of pokemon whose nicknames start with the letters "D" "N" and "A" (one of each, doesn't matter what order they're in). Beat 7 trainers and quit or just lose the eighth battle. Once you're out of the tower go back to the breeding center and talk to the guy outside. He'll say his line about them not getting along or whatever. Then go to Mossdeep and enter the Space Center. Go upstairs and talk to the guy at the computer, and finally he'll say something about wanting to be young to be an astronaut. After he's done, go back out and Surf around Mossdeep so you're up behind the gym. Keep surfing around there and you can catch Tentacool. It's a lot of work, but it's worth it.
