Water is much more dense than air (about 1000 times more dense) so hydrophones do not make very good microphones, and likewise microphones (even if made waterproof) do not work well in water. However, geophones and hydrophones can sometimes produce acceptable results when interchanged, notwithstanding the fact that hydrophones respond to pressure but geophones respond to the derivative (i.e. can't respond all the way down to DC).
*[[Finite state transducer]]
* J. Allocca and A. Stuart, ''Transducers: Theory and Application'', Reston 1984.