unboiled water; raw water
Refers to water that has not been boiled or purified, often with the implication that it may be unsafe to drink.
You shouldn't drink unboiled water when abroad.
Please avoid gargling with unboiled water.
Compound of 生 (nama, 'raw, unprocessed') and 水 (mizu, 'water'). The reading なまみず is a straightforward combination of the kun'yomi readings of the two kanji.