I just beat number seven gym what do I do next?

You get a Pokemon that knows dive and search around the seafloor for a large area that looks like it has no bottom. You will find a door. Go through and you will see the submarine that team aqua pinched.

Go to the surface and make sure some of your Pokemon know strength and rock smash. Find your way around seafloor cavern and you will find archie. Battle him and then kyogre will fly away.

It will start to rain like mad. Go to the cave of origin in sootopolis and find kyogre. You can battle it now.

It's on level 45.
Kyogre's moves are: Hydro Pump, Body Slam, Ice Beam and Calm Mind.
It's a good Pokemon, I hope you catch it...I didn't...sob! I have Groundon though.
Do NOT use the masterball on it. Save it for Latias or Rayquaza.

Good luck!

Well first you'll need a pokemon with Dive, you'll have to go and find Team Aqua. They'll first be in Slateport, then you have to follow them to an underwater cavern.

It should be around route 126 or something like that. Look for a big stretch of dark water.

There should be exactly two different stretches. One leads to Sootopolis, where the eighth gym is, also where you'll catch Kyogre.

(by the way, don't use the master ball, use it when you run into Latias. And don't use it for Rayquaza either, I know thats asking a lot, but trust me, it'll save you a world of time and annoyance.)

The other leads to the uinderground cavern, which is where you'll find Team Aqua. And you'll need top defeate team aqua before you can catch Kyogre.

Beat the 8th gym, beat the Elite Four, get all the Pokemon, win the Master Rank Contest, then you beat the game.