Yeah, I'm guessing these are special Reimus. Maybe if you coat them with thin layer of wax or some other water-repelling substance they could survive floating.
There's nothing really special about them. They haven't melted (most of them) yet, as they might have only been in there 5 seconds. It's not like they start to melt away instantly either.
The artist probable felt that a creature that can drink water would have at lest normal water resistance. None of them are melting, the Reimu that is underwater is only there because she can not hold air due to her wounds.