The 0.3,0.6 and 0.9 are actually log density units, which are used in sensitometry for things like defining characteristic curves of film.Lee mark their's as 0.3, 0.6, 0.9 etc - they work in EVs.
But what I am looking at filters that dont even have stop numbers on them do they???
What ND be best for water then as i dont want to buy alot of filters would a 2 stop be good enough or would it be best getting a 3 stop?