I'm glad the Mario comparison came through; that playful quality was definitely part of the intent. And you are right about the theme drifting more toward underwater or beach than lake lol.
Funny you mention the keys idea; The two pianos—the reverberated piano that runs throughout the song, and a second one appearing from 1:10 to 1:30 and again from 2:12 to 2:32, are quite quiet in a way. Funnily enough, I originally had shimmering syncopated chimes but thought it might've been too much. I still have that track, just muted in the project tho haha.