I’m the photos one can clearly see a line where part of the object was out of the water, since there’s no barnacles above that line. Also, if one were to read the article linked, one would read

(Italics mine)

"Sometime yesterday, a local lady and her partner discovered it just floating on the edge of the water and dragged it out with their four-wheel drive," he said.

That's true, however you can see from the barnacles that this object is in fact buoyant.