An inflatable raft would not be able to be inflated at the depth Dr. Daughtery and Rich were at. The pump would not have been able to provide enough pressure to fill it against the water pressure at that depth, and even if it could, the boat would just be crushed. Assuming the boat could actually be inflated, it's extremely unlikely it would do so instantaneously and that it would be able to lift the submersible as well as the still attached "nearly 2 tons" of cable.