Possible Clues:

• Noises
• Auditory stimuli
• Aural stimuli
• Measures depth or quality at bottom of sea
• Sees how deep the notes are?
• 'Thwack!' and 'Grrr...' e.g.
• Bugs pick them up
• One hears that the waters are narrow with 31 across
• Thus one finds how deep the noises are
• '___ good!'