A hydrophone is exactly what I was planning on using. Marelux has a camera mounted hydrophone that uses an M24 fitting for the through hull port. It's that big for monitors using an HDMI cable, so the hydrophone is the same size. I'll need to get my recorders in before I proceed, so I know how big it actually is, but I'll reach out when I do!We don’t do the holes ourselves but others that have used the XL for hydrophone deployment (at 800 ft) drilled out a hole to pass the hydrophone sensor/microphone
I’m not sure what they used to seal it up but if interested DM me and I can inquire. The canisters are anodized aluminum so quite easy to drill through the cap or bottom.