4. Water As A Weakness
Bruce Willis' character David Dunn has a weakness to water, it is his kryptonite so to speak. His superhero garb seen at the end of the film even includes a raincoat, relevant obviously because it protects him from water. This is not the only time that M. Night Shyamalan has used water as a weakness in this way.
Signs, which came out in 2002, two years after Unbreakable is a science fiction horror film featuring an alien invasion. At the end of the film, it is revealed that the aliens are vulnerable to water, which the main characters use to defend themselves.
This familiar theme of water could be a simple coincidence but it is certainly interesting enough to make note of.