He had not quitted for an hour, during her adventure, the shore of the mystic lake; he had on the contrary stationed himself where she could signal to him at need.

Her need would have arisen if the planks of her bark had parted--THEN some sort of plunge would have become his immediate duty.
His present position, clearly, was that of seeing her in the centre of her sheet of dark water, and of wondering if her actual mute gaze at him didn't perhaps mean that her planks WERE now parting.

He held himself so ready that it was quite as if the inward man had pulled off coat and waistcoat.

Before he had plunged, however--that is before he had uttered a question--he perceived, not without relief, that she was making for land.

He watched her steadily paddle, always a little nearer, and at last he felt her boat bump.