Have you defeated the rival at Bond Bridge and fought through Berry Forest? Once you reach the end of Berry Forest you speak to the Berry Man, who then heads back to see Prof. Redwood. You return and speak to Prof. Redwood too who then informs you that the former Navy Supervisor who lives in the house with the red roof is willing to give you a Tri Pass. Once you get the TriPass you should be able to head into the Three Isle Port Porthouse and go to Two and One Island. The Cave should become unblocked once you get your first Pokemon.