The bikini Yuki is wearing is the sacred bikini from Dragon Quest III (possibly some other ones, but that's the only one I know for sure it's in).

I think Ryouko's robes are the robes of water from the same game, but I can't confirm that.

EDIT: Just checked my guide. They are definitely the robes of water (or as the guide calls them, "Hydro Hagoromo")