There is no such thing as a perfect defence, against an enemy with unlimited resources. But it is much more expensive to attack an enemy's power plants with $1 million cruise missiles than it is to cut their underwater cables by dragging anchors over them. Also, you can plant bombs on underwater power lines and set them off later - that's a huge problem re: how much energy capacity could go down at once.

Anyway, we definitely can protect installations from stand-off weapons: CIWS systems like the Phalanx can shoot them down these days, and they're relatively cheap. It's not perfect protection - eventually one will get through - but it does raise the cost of a successful attack substantially.