That was because of the spring properties, not because of the water. However, water scenarios are a pretty plot convenient situation for the drug to run out.
I like how Momiji is completely aware of Mahiro's ulterior motives. Other times, she can get jealous of Mahiro's attention towards her big sister, but here she's just amused.
The beach wins!!Uh, um... Onee-san...Don't worry about it!I'd like a bit of your advice...!!Miyo-chan, sorry......1 vote for the beach!