How to measure acoustic pressure level under water with a hydrophone?
A hydrophone should come with at least a typical response curve.

The thing to aware of is that that response represents an idealized far field response in an infinite space referenced to an wave of an ideal point source taken at one meter (or yard).

A good reference that covers the conversion of acoustic to electrical energy is


  Horton, Joseph Warren. Fundamentals of sonar. United States Naval
  Institute, 1959.


but his book is scarce as hen's teeth and requires luck or access to a good University library that collected physics and engineering books from the 1950's.

There are other books on transducers.  Essentially an accurate SPL measurement is a bit more than a hydrophone hooked into a sound card.  The field expedient calibration  method is to use a calibrated source at a known direct path distance at least 100 wavelengths in distance. 

In most cases the cable you need is going to be too long for a unamplified transduction signal.  It's best to have the converter at the sensor and send over a digital signal but have weak analog signals close to digital circuitry.

Long cables usually imply higher voltages to minimize resistance losses.  Sharks are attracted to cables that have high voltages so don't be surprised when you see cables offering Kevlar with steel reinforcement. 

Ideally access to pressure chambers to test your the water-tight integrity of your hyrophone cable is a very good thing to have if you expect to dangle more than 20 feet and the deeper you go the more important it is to pressure check your set up.