She believes she slept for several hours.

What an extraordinary situation! Alone in the midst of the vast strait in the southern Pacific, surrounded by sharks, with no friendly sail in sight, and yet slumbering and unconscious.
Of course she was not swimming all this time.

When she found herself growing weary, she floated on her back for long periods, then propelled herself first upon one side and then upon the other, and all the time the dim misty object in the distance remained as far away as ever.
Finally, when she raised her head and looked for it, she was dismayed at being unable to detect it at all.

It had vanished.
Then she knew that it had been an optical delusion from the first.
There was no island or land in sight.

She was alone on the vast deep.
But the heroic woman did not despair.