Even biulding the equipment. There are rules about hydrophones at certain certain frequencies. Just putting the plans online might runafoul of export rules. Beware of stringing multiple hydrophones as this article suggests. Put too many on a system and you are into possible beamforming territory ... the tech used for geolocating noises underwater. The USN gets kinda twitchy about such things.

https://www.ecfr.gov/current/title-22/chapter-I/subchapter-M...

(Search for hydrophone)