Hydroscope is an instrument that shows changes in:

Hydroscope is an optical device for enabling a person to see an object at a considerable distance below the surface of water by means of a series of mirrors enclosed in a steel tube. Hydrophone is an instrument used for recording sound under water. It is the underwater equivalent of a microphone that measures pressure fluctuations, and these are usually converted to sound pressure level (SPL), a logarithmic measure of the mean square acoustic pressure.