The rover was suffering from a hallucination the moment his ship was sinking and he was sure to die. He could hear the bell ringing under the sea, as he was feeling guilty for cutting it and guiding other people to danger. The poet has given the Biblical reference here that it was as if the Devil was ringing his death knell and calling him to the hell for his evil deeds. This is called poetic justice that Ralph the Rover deserved for his deeds.