An article by W. George Schmid in The Hosta Journal  (1996 Vol. 27 No. 2) states that This is a duplicate, incorrect or outdated name for Hosta longissima 'Nikazaki Mizu'.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.