This is a hint on how to get on the 2nd island without unlocking it you go underneath the pillar by the construction site on the bridge (it's easier to go in the water lol) then you swim to the second pillar and wait for the tide to fall and then you press square. And as you know vance will jump on the pillar and carry on throughout the sea and wallah you are on the second island
We have more cheats and tips for this game here, Grand Theft Auto: Vice City Stories cheats
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