A phone would probably be the most useful sensor. You call the people that laid the internet cables and ask.
Russia here, can you tell me exactly where that cable is so I can do a mid ocean fiber tap, ya, thanks.
There are many clandestine reasons for finding deep sea cables.
Indeed. Croatian coastline. Possible game of phones?
Try using OpenCV on some images of stretched uncoiled rope and attempt to segment out the rope if you can. You will get the idea pretty quickly. Think ultrasound, that sort of thing.