A lot of cables are underwater already. The wiser path here is, of course, to replace the threatened cables with those designed for underwater use. Easier to do that than try to change the planetary climate, one that naturally has sea level changes of hundreds of feet.
Even that is not necessary. How many Internet cables from 15 years ago are still in use today? First, I don't believe cables that are not safe for underwater use were placed so low that a few inches of hypothetical change in sea level will flood them. Second, if this were real, we could just place new cables in a higher location as capacity is added. All existing cables will be obsolete in 15 years (assuming we are even using cables then).