Cables can be buried and ploughed into the sea floor. This is usually done in the shallow last miles when approaching a landing on a coast, because there the risk for damage due to anchors, fishermen and other human activity is far higher. However, sometimes the ground can be unsuitable, and burying is expensive, so this isn't done for the whole length.

Doesn't need to be a drone, there should be coast guard etc of all neighbouring countries nearby that can dispatch a plane.

That said, radar systems and sattelites should be active at all times too keeping track of every ship on there, especially if they don't have a transponder active.