When the whale began pulling Nan away, she wasn’t the only one in the water.
Her diving companion, on the other hand, lacked her experience and didn’t share her grasp of cetacean behavior. They didn’t see anything was wrong, and Nan couldn’t express that she needed help because she was attempting to keep her movements calm.
Even if she had been able to do so, the second diver would have been unable to assist.
Nan was on her own.