Daphne in the Brilliant Blue (2004)

Original title: 光と水のダフネ

In the future, water has covered much of the Earth due to the effects of global warming, leaving the human race to live on neighboring floating cities. The orphaned Maia Mizuki, 15, just graduated from middle school and has already applied for employment in the elite Ocean Agency, part of the futuristic world government. Only the best, most intelligent, and physically fit students are eligible for admission.