The verb drown means to die from inhaling water into the lungs and being unable to breathe. It is possible to drown in a small amount of water — even an inch or two in a bathtub.
If you kill people by submerging your victims in liquid (especially water) until it fills their lungs and asphyxiates them, then you drown them. You can drown things in a less literal way as well. If you drink alcoholic beverages to forget your troubles, for example, you can say you're trying to "drown your sorrows." If you're submerged in a tidal wave of school assignments, then you are "drowning in work."