John Constantine explains that water acts as some sort of lubricant to facilitate the transition into the hell dimension. However, I don't see how exactly this works. In particular the film left open some questions:
- What does it take for a human to make the transition?
Obviously the water is not enough, people have been known to come in contact with water, to submerge themselves in it and even to drown. They don't all cross over, so clearly there is more needed. (Note: Even those who "see" would wash themselves and thus come in contact with water. I don't suppose they cross over every time.) We also see that people can be forced to transition several times.
The cat John uses once doesn't seem to be there for the transition but just to locate Angela's sister.
- What is the nature of the transition?
Does the respective person actually cross over or is it just some sort of hallucination? John seemed to have the same clothes and objects when in hell. And related to that ...
- How does one "cut the link" to get back?
When we see John trying to get out, he smashes what can be assumed to be a flask of holy water against his chest. Is this necessary? If so, how does Angela manage to get out? She didn't seem to have any holy water on her person.