I did a quick search for landuse=water on overpass (https://overpass-turbo.eu/s/1Zg4) and it came up with a total use of 3 world wide. That makes me think that landuse=water is definitely not the correct way of tagging :). 

I had a look on de landuse wiki page (https://wiki.openstreetmap.org/wiki/Key:landuse) and there is no mention of landuse=water there. It has however landuse=basin. Which on its own page (https://wiki.openstreetmap.org/wiki/Tag:landuse%3Dbasin) more or less says that that is a synonym for natural=water + water=basin.