"What is it ?" "You mustn't--" shouted Brown.

Then he remembered that he must not shout.

Shouting might awaken the lightkeeper, and the latter would misunderstand the situation, of course.

So he cut his warning to one word.
"Wait!" he called, and began swimming toward her.

She did not come to meet him, but merely ceased swimming and turned on her back to float.
And, floating, the tide would carry her on almost as rapidly as if she assisted it.