She knows the feeling of drowning all too well, but it doesn't scare her, because she can take care of herself.
If she is drowning, it means she hasn't learned to swim. If she can't swim, she will just kick her legs and flap her arms until she gets somewhere because there is no way she will die without her comrades. Those comrades can swim. They won't drown. Therefore, logically, she just won't die.
She knows very well what it feels like to sink to the depths of something stronger and more powerful than she. She knows what it's like to suffocate. But it doesn't scare her.
Because it's in the moments she can't take care of herself that he reaches out to grab her hand, and she isn't drowning anymore.