Hah hah. You reckon Miuna actually fell for Hikari? It's pretty clear she likes him in some way, but maybe not romantically. I hope not. It would be immature infatuation she would never confess, and Hikari would hardly reciprocate either if she did.
The underwater community seems to be finished anyway, so I don't think the words were meant to refer to saving it, but perhaps saving the individuals still left.