It already happens a lot. The cables around Matsu keep getting mysteriously cut definitely not by China[0], and there's nothing anybody can really do due to plausible deniability.

I think the bombing of the Nord Stream pipeline has shown that even when a blatant act of sabotage occurs under the sea, as long as nobody claims responsibility, the debate will rage on for years about who it might have been. I'm not sure if we can really differentiate between state actors and terrorists any more when it comes to this kind of operation. Perhaps it's more a question of did anyone take responsibility, or did no one?

[0] https://apnews.com/article/matsu-taiwan-internet-cables-cut-...