The symbology of this comes from the optical illusion of looking at objects through water. Objects can assume different shapes, may be placed differently from what they appear, or may not be visible at all. Neptune can also represent our psychic self, that part of us which exists in a different medium. Another characteristic of Neptune in the chart is it's tendency to dissolve. Confusion in the area which Neptune affects is a real possibility, again, due to it's distortion effect.
Intoxication is a Neptunian quality which includes alchohol and recreational drugs.
As a matter of fact, people with a strong Neptune are often warned to avoid intoxication as much as possible, and may even need to take additional care with prescription drugs. Also, there can be an emotional susceptibility to despair, common in addicts. Since despair is another version of overwhelm this is understandable. Neptune also rules the dream state, which is essential to our well-being. Fantasies and daydreams are often part of this.