Add a 2D plane. Scale the plane to make it larger. Enter edit mode. Select loop cut. Adjust the number of cuts. Subdivide the surface. Turn on proportional editing and adjust settings. Select and move vertices. Increase the proportional size to smooth the surface. Select and move the vertices. Create a sand-colored material. Select all faces and assign the material. Enter object mode. Add a new plane and scale. Create a sea-colored material. Move the new plane to surround the island. Right click and select shade smooth. Make any final adjustments.