> Surely a lot of communications are within a continent, not needing deep sea cables?
Why? Lots of packets flowing in all types of directions, even if I'm just writing to my neighbor on Whatsapp. I'd be surprised if even 10% of my packets never left the (European) continent. Maybe it'd look differently if I lived in North America.
I would guess that some intra continental communications still use sub-sea cables, either through necessity (UK to France) or to cut out a large number of hops (say South Africa to the Ivory Coast; I don't know if such a cable exists, but I think there are cables going alongside west Africa).
Surely a lot of communications are within a continent, not needing deep sea cables?