That tide did not need any assistance.

Brown swung on his side and settled into the racing stroke, the stroke which had won him cups at the athletic club.
He reached her in a time so short that she was surprised into an admiring comment.
"Oh!" she exclaimed, "you CAN swim!" He did not thank her for the compliment.

There was no time for that, even if he had felt like it.
"You shouldn't be here," he said sharply.
She looked at him.
"Why, what do you mean ?" she demanded.
"It isn't safe.

A little farther, and the tide would carry you out to sea.

Come back, back up to the cove at once." He expected her to ask more questions, but she did not.