It's tempting to say "if it's such a problem why don't they just monitor better?", but the ocean is vast.
I mean putting an officer / escort on board of each ship to make sure they're not doing anything weird would be an option, but I'm fairly sure that's against international law or anything. The issue is also that these are international waters - the Eagle went right through a narrow corridor of international waters - where it's a bit of the wild west.
https://en.wikipedia.org/wiki/Automatic_identification_syste...
However, dolphins don't have as much depth capacity as other whales. I'd choose sperm whales for this, if they were trainable: https://en.wikipedia.org/wiki/Physiology_of_underwater_divin...
It takes a decent amount of coordination, equipment, and crew to pull off these missions.
I imagine many people on these ships aren’t getting paid that great.
By offering levels of rewards for information about what organizations are being used to funnel money and equipment into this, it might help bring some level of prosecution, as well as make the missions more difficult to operate as there will be a lot less trust.
They're dragging anchors on the marine floor; all coordination, equipment and crew are already there as a medium-sized fishing vessel is perfectly sufficient for this job.
We cannot prevent it, we can only detect it and (try to) identify who did it. The planet (some special interest areas) are constantly being monitored (satellites, surveillance drones) and when large ships are not being 'naughty' (switch of transponders) it's relatively easy to put two and two together.
As a side-note, Russia's playbook it to cause small and frequent sabotage(s). Small enough to not cause massive retaliation (i.e. it forces someone to destroy their railroad infra by causing thousands of derailments within one hour), and frequent enough to cause actual damage.
Small sabotage attempts at water towers, power plants etc. Nothing actually happens but it gets in the news and people think something COULD have happened.
It keeps people stressed a bit all the time and they start wanting things to go back to "normal" at any cost - even by letting Russia keep the bits of Ukraine they assaulted.
Most likely, they prop it as a huge win domestically, so that the public keeps thinking they are winning and become easier to placate.
Then why don't these sabotages happen in US waters if it's so easy to get away with it?
Neither the EU countries, nor the US have enough ships to patrol or escort every civilian ship that happens to parse over an undersea cable.
Assume that the Russian shadow fleet starts targetting US cables. What would, could, the US do? The US government could easily retaliate by simply opening the US weapons depos to the Ukrainians. Target US infrastructure and Ukraine gets whatever it wants. The EU can't really do something similar.
Technically you're not wrong, if the EU navies where large enough, they could just escort every single civilian ship, but that's not realistic.
E.g. perhaps something along the lines of prison for captains who were 'derelict in duty', rewards to crews who grass and seizing ships that, by dragging anchors 'by accident', have proven themselves unseaworthy etc.
Good luck enforcing that. The "shadow fleet" ships all operate under flags of convenience and ownership is hidden behind layers upon layers of shell companies.
In contrast, the North and Baltic sea are a hodgepodge of individual nations' 12-mile zones, the Baltic Sea operates under an entirely different set of agreements that guarantee free passage, and it's not really deep so submarine infrastructure like cables can be hit by anything from anchors to divers with explosives.
This is only effective because there are no real consequences for it.
What you want is a combination of maritime patrol aircraft and offshore patrol craft. Maritime patrol aircraft (helicopter, fixed wing aircraft, UAVs) are equipped with surface search radar and other sensors that enable them to monitor a wide area. The patrol craft are also fitted with radars and sensors, and can also move to intercept a suspicious ship. A helicopter could also potentially be used for intercept