Tar-Aldarion (700–1098 S.A., reigned 883–1075 S.A.). He was the sixth King of Númenor, succeeding his father, Tar-Meneldur. Tar-Aldarion was married to Erendis, a Númenórean woman, who was not quite of the same status as Aldarion himself, and therefore had a shorter life-span. Aldarion's love for the sea ultimately caused a rift between the couple. The tale of their marriage and estrangement survived the Downfall of Númenor as "Aldarion and Erendis, or The Mariner's Wife".
Tar-Aldarion was called "The Mariner", and his name signifies "of the trees" because he was a noted tree steward and forester — trees were essential to the construction of his fleets, but he took great care to replenish what was being felled. His first love was the sea, and he was already a great explorer before he became king. He made several journeys to Middle-earth and established contact with Gil-galad and his realm in the Grey Havens. He also established a vital Númenorean port in Middle-earth, named Vinyalonde (New Haven). He built the great haven of Lond Daer on the river Gwathló, and under his reign the deforesting of Enedwaith and Minhiriath began.
His only child was a daughter, Tar-Ancalimë, and so he changed the rules of succession, replacing the principle of agnatic primogeniture with that of fully equal primogeniture. She became the first ruling Queen of Númenor. If the law hadn't been changed then the throne would have passed to his sister's son Soronto, heir presumptive at the time.
The partial story of Aldarion and Erendis can be found in Tolkien's Unfinished Tales, (ed. Christopher Tolkien). Though the actual story was left incomplete, the book does give some hints on how it continued. The story also contains the first hints of the return of the Shadow, i.e. Sauron, to Middle-earth.