you must wait until the ID of the island is the same as the ID of one of your Pokemon.you have to trade a lot.(every day the ID changes).
This is compleatly random. you can expect to see it once. on the island there is a berry that is onlyon the island and cannot be buried to grow more. (not even berry master wife)
also you can get latias in ruby or latios in Sapphire.
"also you can get latias in ruby or latios in Sapphire" NO, you can't. you can only get them(i think in emerald only) after beating the elite 4 and randomly searching.
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Person who doesn't know the meaning of Pokemon Sapphire and Ruby.
In Pokemon sapphire you get Latias. In Pokemon ruby you get Latios. In Pokemon emerald you choose after you defeat the elite four. In Pokemon sapphire you get kyogre and rayquaza, in Pokemon ruby you get groudon and rayquaza, in Pokemon emerald you can get them all.
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you can also get rayquaza in sapphire . hes in the sky pillar on the top which you will need the mach bike to get .