it doesn't exist so don't bother.
fairyland grassland headland homeland landfall landfill landmark landowner
There is no opposite for a word like 'land' any more than there can be an opposite of nouns like 'cat' or 'dog'.
An island. The definition is "a piece of land surrounded by water." Lake titicaca is a good example, having and island in it made only out of reeds.
The best term for protected land is "Turf", or Autochthonal, or Indigenous Land.