First of all whats the chance of the tether getting snagged or trapped by something while in the water? Second if it does get stuck, does the drone have the ability to disconnect from the tether and float to the surface?
The present standard twisted pair tether that comes with the OpneROV ROV will sink to the bottom as it is not neutrally buoyant and does have a good chance of getting snagged or trapped by objects on the bottom.
At the OROV Guide 5 - Finishing Step 31 you will see that the standard twisted pair tether is hard wired to the wire harness two yellow tether wires and therefore will not allow the ROV to disconnect itself from the tether if the tether gets snagged or trapped by objects on the bottom.
On the other hand the new neutrally buoyant tether will not sink to the bottom and will follow the ROV around unimpeded by objects on the bottom. The Trident comes equipped with the new neutrally buoyant tether, but is connected to the Trident chassis through a connector that probably has to be manually connected and disconnected by the operator.
I assume this is neutral in fresh water?