I had an idea for a story to the games where Umi starts out as a young and crippled child with a fear of water, and learns how to use the hook shot to have above-average physical adeptness. She is an ordinary schoolgirl in one timeline, and part of a resistance group who battles against mutant fish who have enslaved humanity in the other. The sea goddess, who is upset about overfishing, wants to combine the two timelines to punish humans with a flood.
Now, that sounds pretty epic and absurd for an innocuous, plotless game such as this one. I like what they officially came up with, though I like to imagine that's what's happening when I'm playing. It would make for a good non-game adaptation. Still, I like how the game sticks to being a game and doesn't even have a villain (which I think is interesting).