BLUE INFERIOR (Buru Infiria), by Kyoko Shitou. First published in 1998 and first published in North America in 2004.
PLOT:
In the future, much of the land has been transformed into uninhabitable desert and the oceans plagued by poison tides. Humans lived in isolated pockets of habitable "blessed" land, and Liber might be the most blessed by virtue of its beautiful seascape. Kazuya spends his days exploring the ocean in the hopes of finding clues about what happened to his parents, but his explorations lead him to a mysterious girl named Marine. Marine has no memory of how she ended up washed up on the beach, but the longer she hangs around Kazuya the more she remembers. Her memories might be the key to the fate of Kazuya's parents and the world beyond the blessed lands...if a traitor from within the community doesn't stop them first.
