It's important that we don't apply human emotions to this situation. Bears are not humans. We don't know that they have the same emotional responses that we do. The primary rule of the animal kingdom is to preserve your own life, and survive at all costs.
As soon as she sensed that she was downing, this mother's primal urges might have kicked in. None of this would have been much comfort to the cubs. They couldn't swim, and so they had literally no chance of survival alone in the water.