Decompression sickness 

Decompression sickness (DCS) is the result of inert and semi-inert gasses that dissolve into tissues and muscles under pressure coming out of solution. The gasses come out of solution when the pressure is released from the body as one ascends to the surface. If someone surfaces too fast the bubbles come out of solution as large bubbles that bock blood vessels and if bad enough will eventually lead to death. 

This is the cause of death of both Chris and Chrissy Rouse. They ascended straight to the surface after their ordeal within the wreck causing the nitrogen in their system to come out of solution too fast. This killed Chris near instantaneously and Chrissy mere hours later. The severity of DCS within Chrissy was so advanced that during the autopsy the blood within Chrissy heart was just foam.