The distance between land and sea determines the water supply. The water supply of seawater is too far from the shore and has lost its economic value. So, your profit range is the island and the shore. The coastline is very long, and your transportation is limited to the coastline (3 kilometers offshore, inland, without your desalinated water). Therefore, from an economic point of view, this economic benefit and infrastructure investment are not equal.
So, where is the profit source of your wealth?