 FACT: In the last ocean scene, Susan removes her tank and BC, then  pushes them away before slipping beneath the surface. This is  impossible to do as she is wearing a wet suit which has enormous  buoyancy by itself. In fact, she jettisoned her weights from her  weight belt earlier in the movie. It is these weights, coupled with  her tank filled with air ( scuba tanks are heavier when they are  full rather than empty)that allowed her to descend at all. Without  the weights, tank and regulator it would have been impossible for  her to submerge and drown herself.