But he kept his ears wide open, though the little noise made by the oars and the rippling of the water prevented him from hearing anything at first.

It was so dark that one could hardly see another in the boat.

It was in vain that Christy watched in the gloom for the glow of a light; for all was nearly total darkness in every direction.
In about half an hour they began to hear the sounds which had attracted their attention on the island, and they proceeded from directly ahead, indicating that the operations, whatever they were, came from the entrance to the sound.

The workmen were not likely to hear the approach of the boat while they were making so much noise themselves.

In addition to the sounds they had heard before, they recognized the noise of escaping steam.
This last discovery made it certain that a steamer was there, though the listeners could not know whether it was the Teaser or not.