Reddit mentions of Array Signal Processing: Concepts and Techniques

Well I work as a sonar engineer, so I can probably help you there. Something to be aware of, is that higher frequencies will attenuate in the water exponentially faster. So just make sure the ranges that you are worrying about are feasible.

There are plenty of ways to approach this problem, but frequency domain beamforming would be my suggestion. Below is a link to explain how a conventional beamformer works. This type of beamformer is the easiest to understand and can localize a source.

This is what I used to understand the terminology of beamforming and how it works.
Microphone Array Tutorial

The way I would approach this problem would be in the frequency domain. Record and quantize your time series using A/D converter for each phone (must be synchronous, or else it'll further complicate learning this advanced concept). Take the fft of each phone data to translate it into the frequency domain. Then create your spatial filters. These are the w's that the resource I linked will explain. The filter is created based on array geometry, frequency and direction you want to look at. then the beamformer output will be the y(f, theta) = w(f, theta)^(H)x(f). Where x(f) (the snapshot) is a vector comprised of the values of the fft for a specific frequency bin across the hydrophones. If this is too confusing read through the pdf I linked, and focus the most on creating a steering vector. Something to keep in mind is that the resource I linked only explains how to beamform for a linear array, not an L shape. However, once you understand the methodology it is simple to change your weights or your steering vector based on your arbitrary array geometry.

I tried my best to explain this in text. But this is a concept that is has its own dedicated graduate courses. If you have any questions, feel free to PM me or respond. I'll be happy to answer.

Edit: BTW this book Array Signal Processing is the easiest to understand and what I used to learn when I was a senior. I found it my school's library. It dumbs down the concepts and you will only need the first 3 chapters to understand what I'm talking about and how to implement it. Only focus on conventional beamforming now, adaptive beamforming would be too much to tackle if your inexperienced.