Cable-laying ships have been around for many years, as have undersea cables and fiber optic links. Last evening I watched one of the "Mighty ships" episodes that showed how they connected Costa Rica to the existing Pac 1 fiber optic link between California and Panama. After laying the new cable, they cut the existing Pac 1, picked up both loose ends and inserted a splice, before setting it all back on the ocean floor.
All underwater operations were carried out remotely, with no divers in the water. A 'plough' was dragged behind the ship to bury the new cable until they were in sufficiently deep water. A 'blade' was dragged through the (deep) water to cut the existing cable, although they only had a rough idea of where it was. Then a remotely operated submersible used high pressure jets to re-bury the cable.
The first test of the new cable failed but, on investigation, they found a damaged cable at the beach end in Costa Rica, and someone breathed a huge sigh of relief.
We take a lot of this stuff for granted when we surf online daily.