The power cable operators don't want to waste any energy and will have power monitoring systems at least at both ends, so much as with a fusebox at home, I'm sure that the cable would be switched off very quickly if that happened. In addition, the power would be dissipated close around the source of the breakage by the (not insignificant) electrical resistance of water. Since the North Sea is about 100 metres deep, the only victims in such event would be fish and other seabed creatures. I don't tend to chew power lines, though, so don't worry about me.
