An article by W. George Schmid in The Hosta Journal  (1996 Vol. 27 No. 2) states that this is an "old" incorrect name for Hosta longissima 'Nikazaki Mizu'.

Mikiko Lockwood in an article on The Hosta Library titled, A Little About Japanese Hosta Terms defines the term mizu as water or 'Mizu Gibōshi' which is the Japanese name for the species H. longissima.