Twitch/Bilibili-era streamer slang. The
水 is the same one in
水帖 (low-effort posts) and
灌水 ('flooding' a forum) — viewers in chat are 'flooding the stream' with
弹幕 and comments. Hugely common in
直播 culture; a streamer addressing chat will say
各位水友 the way an English streamer says 'chat'.
