As with humans in space, the real question is, why send humans to do a robot's job? Most work involving undersea cables and wells uses remotely operated vehicles, not human divers.
Robots are still very clumsy. I've seen videos of researchers taking minutes to do basic tasks like "yank on a lanyard attached to the ROV" using a robot arm. Oil and gas companies use a lot of robots but they still employ divers.
Humanity expanded for the last ten thousand years but now all the easy places to expand to are full, making Earth into a prison some people dream of escaping. Sending robots won't help.
