[btoptical]

In some cases the cable is buried on the sea floor by a special plow. It's incredible that the cable can be buried up to 6m deep in the mud on the bottom. Cable burial is intended to keep fishing equipment from snagging cables laid on the bottom.

http://youtu.be/QRwZ1hlj9F8

[toomuchtodo]

Jets of water are used to bury the cable closer to shore, and if I recall correctly, Google used Kevlar on their latest cables to reinforce them against sharks.

[boyaka]

That's in shallow water, not answering his question.