You gan get Kyogre in Saphire in the Cave of Origin Basement and in Emerald in the Marine Cave. Eh..hem... Finding him is easy, but catching him could be a chore and a half (if you get where im coming from here) You're gonna need a Poke with some good SPD to wall him, or good Physical attack (like Blaziken, risky but worth a shot) Make sure you bring all of the healers, ultra balls, and other stuff that you need. I also reccomend grabbing yourself a Tentacruel (Using toxic will help) Heres my strategy for you. Other tips: Paralys and sleep work if you can get the right movesets.