This interesting article on SV Seeker discusses the various ways of making a tether for a remotely operated vehicle (ROV). They experimented with a number of different cables, including gel-filled ...

Building an underwater remotely operated vehicle (ROV) is always a challenge ... Like most ROVs, it uses a tether for communication, which in this case is an Ethernet cable with waterproof ...

Rather than building another ROV, the team will design and build an Autonomous Underwater Vehicle, that operates without a tether like the ROV does, adding a layer of difficulty to the process.