Can someone explain to me what the deal is with the water coffins in the movie The Box? I mean, what is their relevance in the film? What is the point of having them there in the library? That scene always seems so isolated from the rest of the movie to me. I understand the ending, I understand the film, but the water coffin scene makes no sense to me.
I think a lot of the movie didn't make sense, but I'm going to try and answer it. Apologies in advance, because this is a long explanation and takes quite a while before it actually even tries to answer your question!
I believe the movie largely revolves around the idea of free will. Do we have it, or is everything controlled?
Take the Box for example. Diaz presses the button, they receive their money and someone else dies. This is their choice. They could either collect the money and live with that knowledge or reject it. It is up to them... or is it? Did they have any choice at all, or would that action have always occurred? Note that the man shoots his wife at the same moment she pushes the button - but there's no indication this wouldn't have happened if she didn't. Instead, it's presented almost as if they simply had to happen. There was no other choice.
We also learn that the woman who was killed in the above scene had earlier pressed the button - she was killed by her husband after he was asked to choose between wife and child. Similarly, at the end of the film, when Diaz is killed, her husband is given the same choice.
So the entire synopsis of the movie revolves around free will and choice - whether we have it, or whether everything is controlled from the start. I don't think any answer is ever given, or intended to be given. It's entirely up to you, the viewer, to decide what you believe happens.
All the random people with nose bleeds in the movie (all working for Steward). Did they have choice, or did they simply act in a way that was predestined by Steward and the higher power. Even Steward can have this question asked of him. He even indicated he was just a messenger and nothing more - he was being controlled by whatever the higher power was after his accident (whether aliens/something else, we don't know).
To me the water coffins are symbolic of this choice. Two lead to eternal damnation, one to redemption. He was given the "peace" sign a few times in the film. Did this lead to him choosing door 2? Was it free will? Or was it destiny? Did he ever have a choice? Is this true of humanity as well? Do we have a choice regarding our salvation?
This fascinating Yahoo Answer goes an interpretation of the movie. To quote from the end of it:
As to your question (I hadn't forgotten!!), using this logic and the idea that a higher power is testing everyone, it could be argued the three coffins scene was indicative of baptism and being reborn. Marsden chose to follow the signs (obviously coming from the higher power) rather than go it on his own. Remember, even though Diaz tried following them as well, she gave into the temptation and pushed the button, not him. He even tried to back out of the deal.
On a final note, I do have to add that all this is just mass speculation. There is no real right or wrong answer and I think it's perfectly plausible to say that the effects looked quite cool and were quite likely added for no real reason beyond that. However, hopefully this answer has made a go of providing one possible (albeit convoluted) suggestion.