Watch Tower
In the not too distant past the southern coast of Spain was prey to "Barbary" pirates whose home base was in Algeria. Thus, the Spanish coast is dotted with towers, where the local people kept watch.
This tower is at the far end of the La Manga sand spit, on the seaward edge of the shallow inland sea which is known as the Mar Menor. Since no one lived on the spit the tower must presumably have been built for the protection of the adjacent salt pans. Either that, or the guard, on sighting the pirates, would sail across the Mar Menor and raise the alarm inland.