The watery fluid of the lungs is a surfactant produced by type II alveolar cells which is mainly composed of lipids such as lecithin and sphingomyelin and proteins. The main function of the surfactant is to reduce the surface tension in the alveoli to prevent spontaneous collapse or atelectasis.
The clinical significance of this is that premature babies are not born with enough surfactant therefore their lungs collapse and they develop a disease called respiratory distress syndrome.