Sailors on tub needed to soften water (4,5)
I believe the answer is:
bath salts
'needed to soften water' is the definition.
(bath salts are used to soften bath water)
'sailors on tub' is the wordplay.
'sailors' becomes 'salts' (salt is a kind of sailor).
'on' says to put letters next to each other.
'tub' becomes 'bath' (synonyms).
'salts' put after 'bath' is 'BATH-SALTS'.
(Other definitions for bath salts that I've seen before include "Perfumes" , "tip these into the water" .)