I think one of the big challenges at the time was how to install the hydrophones, although as I recall there was also a novel type of hydrophone being used. AT&T had invested a lot of effort into figuring out how to not only build long cables that would survive in undersea conditions, but also deliver power on those cables to active equipment (repeaters in the case of undersea telephone cables, hydrophones in the case of SOSUS). This involved putting several-kV (I think into the tens of kV on long cables) DC onto elements of the cable, and it was hard to design a cable that was reasonable to lay but could take that potential without dielectric breakdown. Remember this was in an era where paper was still a popular insulating material on communications cables, if not lead. DC had to be used instead of AC because on these extremely long cables the capacitance between the two current-carrying elements would end up eating up most of the power you put into it.

Between Bell Labs and Western Electric, AT&T had a lot of practical expertise in designing and manufacturing some really complex cable bundles with high voltage and sensitive communications pairs nearby. This pretty much all became obsolete as soon as fiber started taking over in the '80s, but it was pretty incredible how many coaxial pairs AT&T was cramming into a buried cable (along with power for all the en route equipment!) in the '70s. Hell, AT&T famously held off on fiber for years because they had a plan to bury long microwave waveguides like cables!