"and we owe it all to the flopping of the Thing, and the good axe of the Woodman!"
Flopping around like they hadn't any bigger business than to--Reckon they're looking for us to come back, Bruno?"
It is possible that the white-fish's spinal column and flopping tail had interfered with his vision, and in launching himself he may have mistaken the dark, round opening of the cistern for its dark, round cover.