He did not understand the significance of "croaking," but the word had an ominous sound.

It referred to something the man called Bill wished to do to him and something to which the man called Hank objected.

He understood, however, the threat to throw him into the Bay.

The fellow Bill wished to do this while Hank was determined to prevent it.
Instinctively Jamie felt that Hank was only defending him in order to protect himself.

He had no personal interest in him, but did not propose to be involved in any trouble that might arise through some action that Bill wished to take.