Brian Wilson of The Beach Boys wrote this song while depressed and contemplating life and death gazing at the ocean. Its mood is indescribable and is one of the most uniquely emotional songs I've ever heard. Put on a good pair of headphones or turn every distraction off and crank your hi-fi.
This was released late in 1971 and late in Brian Wilson's period of genius songwriting. It appeared on the album Surf's Up just before another brilliant song of his, the song "Surf's Up."
By the way, the cover art for Surf's Up is a painting of a modern sculpture masterpiece, The End of the Trail by James Earle Fraser
Wikipedia's article for 'Til I Die is one of the more interesting wiki articles for any song.