Luna is dumb enough to use logic, even though logical solutions aren't welcome in Gensokyo.
HELP!I'm coming to save you now!Then why did you come!?HELP! Can't you just fly? S-Someone help meeee!But I'm a nagashi-bina, so I can't swim!
Hina is based on the concept of nagashi-bina, little dolls floated downriver on Hinamatsuri to take the year's misfortune with them.Nitori's in trouble!Kappa Getting Carried Away by a River
This is a well known Japanese idiom, meaning "anyone can make mistakes." In this case, it's more literal, 河童も川流れ "even a kappa can get carried away by a river".