Oh god, I'm totally going to get cancelled for not liking this iyashikei classic.
ARIA, by Kozue Amano. First published in 2002 and first published in North America in 2004.
PLOT:
It is the 24th century and Mars has been transformed into a watery paradise. Its prime attraction is Neo-Venezia, where the undines guide their gondolas through the canals to ferry people across the city. We follow the newest trainee, Akari Mizunashi, as she meets new people, enjoys the everyday pleasures of life, and grows ever closer to becoming a full-fledged undine.