泥水
muddy water; mud; masonry (craft)
逆水
against the current; upstream
逆水行舟
lit. a boat going against the current (idiom); fig. you must work harder
逆水行舟,不进则退
like rowing a boat upstream, if you stop moving forward you fall back (idiom)
学如逆水行舟,不进则退
Study is like rowing upstream: no advance is to drop back