Sand absorbs a lot of energy. 3-feet of sand will absorb a lot of energy. The cables are already protected to absorb a lot of energy. A torpedo is only going to move at 200 MPH.
Relevant news:
http://www.bbc.com/news/world-europe-34797252
Seems that Russians are already working on new generation torpedo. It seems it would be aimed more at oil and gas pipelines than internet cables, but who knows...