Thats super cool for the ROV aspect. In university I was part of the underwater robotics program, and optimizing the tether was a huge aspect. We used a fiberoptic line for communication, leaving the rest of the cable mostly dominated by the copper conductor for power.

Any weight reduction that can be made for the cable internals is actually two fold. The cable needs to remain neutrally buoyant as to not pull on the ROV and not restrict the pilot. Less internal weight in the cable means less foam/other material needs to be added to offset the negative buoyancy.