It comes from the Latin words for "almost island."
it is a land covered with water from three sides,
The word is from the Latin 'paene insula' meaning 'almost island'. Look at a map of Spain and you will see why it is called a peninsula.
The Niagara Peninsula is not a true peninsula. It is an isthmus, a narrow strip of land connecting two larger bodies of land. The Niagara River does not count as a body of water, thus the "peninsula"...
That honor goes to the Mexican state of Baja California Sur, which averages over 80 °F year-round.