Liquid bowel movements are common and most people have them every once in a while. Usually they are not chronic and the only danger from having watery stools is liquid loss and dehydration which can actually be life-threatening.
If the watery stools persist, the condition is usually referred to as Chronic diarrha and it could be due to a number of different digestive or environmental causes.
Infectious and becterial diarrhea is often curable, but with quite severe bouts, sometimes called explosive diarrhea that exhausts the person digestive system to the point where so much of the excreted material is water that the poop floats, which is called steatorrhea. Parasite infections are often contracted in unclean environments, usually with contaminate water. For that reason these disorders are often referred to as Travelers Diarrhea. Common viruses are the Norwalk Virus, Giardia Lamblia, and Entamoeba Histolytica.
A number of genetic digestive tract disorders like Ulcerative Colitis or Crohn's Disease can cause loose and watery stools. The diarrhea usually is due to imbalances of different bacteria in the digestive tract.