How can I dive in lost in blue 2

you need first go to the last beach. at the exit of the ruins, there will be a beach, with a place where you can build treehouse. then, you go to the beach when the tides out. then, go to the place where you just can only there when the tides out, and you will seems to trigger an event. from then, after the tides out on each beach, you can dive in the spot where you can go there on tides out only.