How is digging a depression supposed to create an island? What mechanisms are you imaging at work? There is a lot of context missing here. Sustainable how? All real islands eventually erode away, once whatever forces formed them stop.

It is supposed to create an Island if the sand would be piled up into a "mountain" above sea level. Sustainable in the sense of causing significant damage to sea-creature species in any way whatsoever (at least if this would be done vastly, say, creating many island in front of shores of small countries such as Lebanon or Israel).