[He's certainly flirted with her, but never thought that it'd go anywhere beyond friendly. That's the biggest reason why he's never tried to make a move... But there was definitely a crush! She was floating there and waiting for him to speak up and after a moment of blushing, Kotarou clears his throat awkwardly.]

Y-You too? I didn't think you'd feel that way about me... I never made a move on you since I didn't wanna make things feel weird. [A beat.] But I really like you too, you know? Being a mermaid would explain why you're prettier than the other girls in class.

[He'll be taking a step closer to bend down to her level while he's on the shore and he offers her his hand to hold.]

So, uh... I'd be really happy if we could go on date and you'd be my girlfriend! Sure, you're a mermaid and I'm a human... But I'm not gonna let something like that get in the way of my feelings!