She became hyperthermic because she was the only one not wearing a wet suit. When you become hyperthermic, the extremities (your arms and legs) shut down first to conserve in your torso what little body heat you have left. It's the body's way of going into self preservation mode. In her case, her entire body was suffering from lack of warmth from only having wet clothing while the others had wet suits.
I believe you mean hyPOthermic: extreme low body temperature. There is also a term hyPERthermic, but that means extreme high body temperature, like from heat stroke.
I initially wondered the same thing, but then realized it had to do with her not wearing the suit. After all, the movie is pretty realistic in its diving and caving portrayal in other ways. Thanks to the above poster for clarifying why.