The cables were already underwater, what's a few more feet of storm surge to them?
Well the buildings that they land in may not like water that much. =)
I believe the cables are dug underground as they approach land. It would be too easy to cut in shallow waters otherwise.
It's progressive, going from just anti-shark/power carrying sheath in the deep ocean to light steel cable armor a ways out to trenched as it approaches the shore itself. They splice the cable together according to a specific plan based on a survey (first desktop, then underwater) to account for underwater terrain and risks.