Just as another data point, the Nereus[1] explored the Mariana Trench at nearly 11km depth, and had a comms tether (very thin optical fiber). According to the wiki, its tether was 40km in length total. So, it's not out of the question, I don't think.

No good for pulling it to the surface, of course, but reliable communication alone would be a great benefit.

[1] https://en.wikipedia.org/wiki/Nereus_(underwater_vehicle)