Slight scale issue here - individual cables are longer than that and the aggregate is probably over 1m km - the rest of the world measures this stuff in km :)
The "design life" of a specific cable is ~ 25 years (or possibly that is the time when the initial cable aging guarantees expire)
A map (mainly of the international ones):
Note: cable repairs often take weeks as a ship is involved, so anyone running a reliable service / network on top is likely to want 3 routes to any major node to get to 5 x 9s availability
Finally the Internet is likely to survive significant sea rise, but I suspect some of the locations mentioned will struggle with being below the new sea level - the only ones who take flood protection seriously enough to start working on mitigation seem to be the Dutch...