I was under the impression that cable systems are best understood as constant-current systems. As a side effect of this, they can withstand a power failure at either end, in which case the ground path is part of the circuit. (Essentially "single rail" operation.)

If nothing else, both ends still share a common ground reference, yeah?

https://pdfs.semanticscholar.org/3c72/998ffe9c39a1aa4a9faba4...

Edit: thinking about this more, it seems quite obvious that you need a current return path regardless. If it's not the ocean/earth - what is it?

-You are right; I just looked it up. I guess (not that it makes much of a difference!) I picked this up from a book on earlier days of submarine cables.

Thanks!