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Yes, or else they wouldn't have made him/her/it. See also How do I catch Kyogre in Pokémon

And only Sapphire and Emerald and if you want it in Platinum, Pearl or Piamond just migrate it.

You can also capture it in Heart Gold by getting a blue orb from Mr. Pokemon after obtaining your 1st gen starter from Professor Oak, your 2nd gen starter from Professor Elm, and your 3rd gen starter from Steven