There is a such thing as fluid in the lungs. Normally the pleural space contains a small amount of fluid that acts as a lubricant to the space during respiration. If you have a significant amount of fluid then you may have pleural effusion or some other respiratory issue.
There is no such thing as fluid in the lungs. However, the fluids from the neighboring organs can leak into the lungs. This disease is called Pulmonary Edema or Lung Water. Numerous disorders can lead to accumulation of fluid or edema in the lungs. It can be caused by Heart disorders like Heart attack, Arrhythmias, Endocarditis, or kidney failure, pneumonia, Shock etc. For details see the link below:
I don't think there is fluid in lungs.