I would assume it has more to do with how many intermediate places within the continent you want to connect to. If you're connecting two cities on the African c

I would assume it has more to do with how many intermediate places within the continent you want to connect to. If you're connecting two cities on the African coast, and there's really no infrastructure in-between that needs (will pay for) connection, and the roads in between are poorly maintained, it's probably more economical to go by sea. Someplace like the US, if you're running cable from say Boston to NY, first of all, you also want to wire up all the towns and cities in between, and you've got interstate highways so your service trucks can easily get to all parts of the cable, you've got existing utility tunnels you can use, etc. In that case, going by sea is an unnecessary expense.