Who needs a sub or diving gear? It’s 100’. Drag an anchor across the bottom at night.
>>Drag an anchor across the bottom at night.
The cables are probably plowed into the bottom and may also be armored to protect against anchor damage.
So you need to drag a plow with a cutting blade instead of an anchor. The cable may not need to be cut. Bending and stretching are not good for optical fibers inside the cable.
Note that all this undersea cable infrastructure is there due to politics and economics. There are no right of ways to be acquired and no transit fees to be paid when the seafloor is used instead of terrestrial routes.