I would suspect that the cable is an electric supply cable. Otherwise, the island would have to generate its own needs. While that’s possible, it’s probably cheaper in the long run to run the submersible cable and buy off the grid.
That is a long way to send electric. Current drops over distance. I would think they produce their own electric with diesel. Hess Oil has a refinery/ operation on one of the US Virgin Islands. Islander told me they ship product to P.R., then it gets sent back to the Virgin Island for distribution. Never made sense to him.
One a side note, he said large planes land in P.R. and de board to smaller air craft or boat, for unnoticed arrival. ST. Thomas has shipping container port for (sealed)container unload and ferried to smaller Islands.
Everything arrives by air or boat.