The island is rocky and set in a sea of sand. Dunes from various size are covering all of the desert, they move with the wind. In between those, water is always present, its level changes regurlarly, it's a tide system. The water level can vary extremelly, but more often is stable as shawn above.

Also holes can be found between sand dunes, they are pierced in the rock under the sand, they never dry up. They lead to vast cave systems forever under water.

You can say it's an ocean of water under an ocean of sand dune!