Once he saw a broad piece rise to the surface together with a quantity of bubbles.

None of the sandbanks now rose more than a foot or so above the surface, and were entirely bare, mere sand and gravel.
The mist ahead was sensibly nearer, and yet it eluded him; it was of a faint yellow, and though so thin, obscured everything where it hovered.
From out of the mist there presently appeared a vast stretch of weeds.
They floated on the surface and undulated to the wavelets, a pale yellowish green expanse.

Felix was hesitating whether to lower his sail or attempt to drive over them, when, as he advanced and the mist retreated, he saw open water beyond.

The weeds extended on either hand as far as he could see, but they were only a narrow band, and he hesitated no longer.

He felt the canoe graze the bottom once as he sailed over the weeds.