The Islet Mystery 15

The sea level started to get high by the time Shiela drifted off to sleep. During high tide, the rise of the water level would cover Chara's rim. On aerial view, the inequality of the water level around Chara and the surrounding body of water could not be identified, and it's elevated position, which looked like a basin on top of the shore, would not be evident.

While one's buoyancy would still retain on the area during high tide, the risk would pose during windy days, low-pressure weather and stormy hours due to big waves. The islet that Ryan owned was in the middle of the sea, and no swimmer was good enough once caught up in big waves due to gravitational pull.