Punchdrunk said: More interestingly, that Reimyu isn't turning into a pile of mushy paste. Magic?
Well since yukkuri is a "fan work", there's no real canon. Most references to yukkuri in larger works are just being cheeky and they don't really concern themselves with the lore of yukkuri abuse world.
I think they only start dissolving after they've been in too long. Also maybe this is a special floaty Reimu that has hydrophobic braids that allow it to stand on water.