A world where, instead of aging in years, the people age in knowledge. So, instead of dying once a general age was reached, they die once they achieve a certain amount of knowledge. A man could live forever if he never learned anything, or could die after reading a few choice books. Of course, the knowledge of the land would be regulated, like medical drugs.
"Look, lad! You see the streak of glowing green off the larboard bow? Them's the bright waters!"
While the lad peered intently at the eerie glow, the old salt continued. "Entire ships have been lost to the Bright, lad, for once you let it surround you, you'll never touch land again!"