The thing that shocks me is how recently these were added. I was checking out the cables in Australia, expecting them to have been added in the 60's or something....however most are around the year 2000-2010.
Seems like each cable generally only has a life of 10-20 years.
Plus, the impact from burning the fuel it would take to have a ship pull up a cable would be thousands of times worse than anything the cable might do.
I can't imagine what materials they would be made of that would be particularly harmful. My sense is they're mostly constructed of things like rubber, plastics, other polymers, steel, copper, and of course a little glass fiber.
As time passes, most of the cable is buried deeper and deeper under debris that falls on the ocean floor. This means that you cannot just pull it up, you would have to dig to get it out.