Characters!

I'm really in love with the idea of there being a little sister archetype. Not as a love route, but for the sheer cuteness factor.

If we're going for the school as a setting, we should probably put some thought into the teachers. It's not necessarily the most important bit, but it's probably worth thinking about.

Perhaps the history teacher is an really old turtle, who reminisces about being the students' age a century ago.

Yes! I mentioned a turtle earlier
they just seem..teacher-ly xd

Right then, in the interests of actually moving things to an editing stage rather than people just suggesting vague ideas I'm going to throw out some broad character archetypes that we can tweak and specify as we go:

Sporty-type: Mako (Mah-ko) the mako shark. A first-year student and the star of the underwater...track...team. Yes. She is friendly and cheerful and generally not very tsundere, however she is reluctant to interact with scuba-kun outside of a superficial distantly friendly level. She instead throws her whole heart into...underwater...track. I dunno, we're gonna have to figure that out. She is, in a sense, a reverse tsundere (Is there a word for that?) in that she is very open and forward and happy but very difficult to get to know beyond the initial stage because, uh, she's been hurt in the past. Or some traumatic event prevents her from really allowing herself to be vulnerable with another person. Shark. Whatever. Perhaps as the story progresses she finds herself having to choose between focusing wholly on some big track meet and spending time with scuba-kun. In the end she comes in like, third, but realizes she's happier being with someone she can trust than being a champion.

Tsundere-type: Samehime (sah-may-hee-may) the tiger shark. Another first-year (hence 'senpai') who apparently takes an immediate disliking to scuba-kun when they first meet. She shows this dislike by periodically trying to devour scuba-kun, however this is a desperate attempt to get scuba-kun to notice her and an excuse to be around him more often. By being aggressive in her hatred it gives her excuses to be in the same room as scuba-kun. Eventually through a wacky series of unusual circumstances she finds herself becoming more and more vulnerable etc etc. We've all played VNs. We know how this goes.

Brainy-type: ? (???) the hammerhead shark. Buries herself in books in order to keep herself from being noticed (Due to her unusual appearance) and therefore doesn't have very many friends. She is, however, very perceptive (As I recall hammerhead sharks have additional sensory stuff in their weird hammer heads) and maybe is the only one who knows that scuba-kun isn't actually a sea creature. However in spite of this fact she finds herself growing attached and eventually attracted to him as he spends more and more time with her and helping her come out of her shell. In the end, however, hammerhead-chan realizes that she's actually happier being an introvert than a social butterfly, but realizes that she never would have come to this revelation without scuba-kun's help. They profess their love for each other and she works up the courage to bite his face in half in front of the whole school. Okay maybe not.

Kame-sensei: The history-teaching turtle. Teaches history that scuba-kun doesn't understand at all because it's all underwater history. Scuba-kun is not doing very well in his class as a result. He is however wise and mentor-y and occasionally helps scuba-kun out with a choice piece of advice.

I'm okay with switching it to an octopus. The basic idea would remain largely the same.

If all the heroines were sharks, then would we still have Nami or no? If not, then maybe would could have a sort of best-shark-friend character who is present in all the heroines' paths and gives Scuba-kun advice or just hangs out with him during their free time. This character could be male or female, but if its female, then she could become a love interest and have her own arc. His/her personality could be friendly at the beginning (which is how he/she becomes Suba-kun's close friend) and then gradually shows themself to be playfully and humorously sarcastic/dry/snarky like Kyon from the Haruhi series.

Nami-san will still be there, and she can be an exception. I'm not saying we shouldn't have non-shark characters, just that those characters won't have their own routes.