That residue is highly concentrated chlorine and yes it makes me cringe.
In order to share with you why the cables are like that let me tell you a story that happened to us.
We had just installed the mini wet con tail modification to the tether and the ROV cameras were really slow. ROV telemetry was ok but everything was delayed. Turned out that we had one pair connected at the ROV and going nowhere and another piar connected on our controller and going nowhere. The reason we had comms with the ROV is purely because of crosstalk because the pairs are not shielded. Took us a while of troubleshooting before we figured that one out.
So you see, non-shielded twisted pairs can’t allow any other comms on the other wires.
This means I have one piece of tether for my rear facing camera, one for the ROV comms, one power cable from the batteries and one bulkhead mini wetcon.
Why do I put my bulkhead there? Because when I install my power module instead of the battery, it will connect to the mini wetcon.
This way I can use the same wetcon either for the battery or my power module. With either tether. So I can switch from a battery operated system to surface supply as fast as just changing the battery…