Originally posted by Roogle
I heard that Leviathan was in early sketches of the game when they were coming up with ideas, but you know why there really is no water Aeon?

Yuna can't swim very well in the first one, and she doesn't battle underwater—only Tidus, Wakka, and Rikku do. Where would she be able to summon Leviathan? On land? That would look a little weird.
Yuna could always ride on Leviathan's back, seeing as how he's a giant serpent.