The Island Cave is a cave that is located north of Dewford Town. The cave contains Regice in the Generation lll games. Once you have opened the caves in Sealed Chamber, go to Petalburg City. Then go to the beach close to it and Surf down and stay next to the left until you see a gap next to an island, proceed through the gap. When you are on the opposite side of the rocks, go up until you see an island. Enter the cave.
For Ruby and Sapphire, wait two minutes and another entrance will open up.
For Emerald, you have to walk around the perimeter of the inside. Then the entrance will appear.