Rare term for mouthwash; the more common word is 洗口液 (せんこうえき).
See also: 洗口液
I looked for 水歯磨き at the pharmacy, but only found 洗口液.
Standard kanji spelling for this rare term.
Abbreviated kanji form; less common but still valid.
Compound of 水 (みず, water) + 歯磨き (はみがき, tooth brushing/cleaning), literally 'water tooth cleaning'. The exact historical derivation is uncertain, but it likely refers to a liquid for cleaning teeth.