I was looking at this back around the Nord Stream stuff.
You can get off-the-shelf small UUVs for like $30k that you could use to plant explosives from a boat.
It’s actually a real issue for pipelines, because there’s essentially nothing by way of treaties protecting them. Companies just kind of started building them, and nobody has really gone about methodically attacking them yet.
For cables, there’s some treaty from the late 19th century, I think signed in Paris, that covers them. Which some countries have signed.