Below are points of evidence supporting the fact that sharks are a threat to undersea cables.
The first report of sharks attacking cables came from the Canary Islands in 1985, when sharks' teeth were found embedded in an experimental cable. I found the report in an old news paper.
According to a report by the United Nations Environment ...
Sharks are an extremely small threat to undersea cable in regards to others causes as shown in the graphic below (from a report cited in @georgechalhoub's answer).
Proportion of cable faults by cause, from a database of 2,162 records spanning 1959–2006
Abrasion 3.7% ;
Geological 2.6% ;
This article is about goldfish, not sharks, but it's the same urban legend:
Goldfish Only Grow to the Size of Their Enclosure
There is an element
of truth to this, but it is not as innocent as it sounds and is
related more to water quality than tank size. When properly cared for,
goldfish will not stop growing. Most fishes are in fact what are ...
The Australian Marine Conservation Society's web site is http://www.marineconservation.org.au/ . Doing a search for shark cull million on their web site gets as the first hit Government on shark watch as cull shelved
Yet despite being essential to a healthy marine environment, the
world's shark populations are experiencing significant decline, with
Yes: the finning trade is estimated to be larger than the commercial shark-meat trade.
According to Global catches, exploitation rates, and rebuilding options for sharks; Worm et al.:
Clarke et al.  used trade auction records from Hong Kong to estimate that the total mass of sharks caught for the fin trade. Estimates ranged between 1.21 and 2.29 Mt (...