1A plant which lives in or by water.
2An installation for the purification, distribution, etc., of water. Also as a mass noun: pipework and other equipment used in providing a water supply.
water plant/ˈwɔːtə ˌplɑːnt/ /ˈwɔːtə ˌplant/
Late 16th century; earliest use found in John Gerard (c1545–1612), herbalist. From water + plant.