How would you trigger them? 100m down radio doesn't work at all. ELF antennas are nation-state level assets and need receiving antennas that are hundreds of metres long.

Acoustic messaging is possible, but again hard to do. Timed charges are plausible but that's a heck of a commitment (of course 3/4 breaches suggests a possible misfire - I imagine theres a hunt on for the last one).