>>131708There's Amanchu coming up next season, with the premise of deep water diving, I believe. I'm looking forward to it. Same author as Aria.
There's also Nagi no Asukara, Blue Submarine No. 6, and Ponyo. I haven't watched the first two, but the other two seem to focus on fish girls, so they're probably close to what you're looking for.
I haven't watched this, but there's also Free! which is all about swimming and animated by Kyoto Animation and, from what I've heard, was specifically made for its animators to learn a lot about water physics and how to animate water/swimming, just in case you want some tasty animation.
A Letter to Momo might fit your request as well, but I don't if it's what you want exactly.
Also, despite its long length, there's always One Piece. Though, the inner parts of the arcs don't really focus on the water much at all, the journeys between islands will definitely get you going.
Taifuu no Noruda, maybe?
Definitely Tsumiki no Ie/La Maison en Petits Cubes.
Porco Rosso is sort of equally 1/3 land, 1/3 air, 1/3 sea, but I feel like you might still enjoy it.
Also, this thread made me realize there are no anime about surfing. Eureka Seven, Space Dandy, and 5 Centimeters Per Second all have scenes where characters do it, but not entire anime dedicated to them. Strange.
