This is the 1139th most frequent Korean word.
물이
"물이" means "water."
"물이" is functioning as the subject in describing the condition of the water being clear.
물이 맑다.
The water is clear.
"물이" acts as the subject indicating the substance flowing over there.
저기 물이 흐르고 있어요.
Water is flowing over there.
"물이" is the subject emphasizing the requirement of water in this context.