Hi @jsnyder, this is something that we're actively working on. We're fairly far out from releasing a product though. There's a lot of things to think about for something like this.
First of all, the tether only has 26 AWG wires, so you can't do anything useful with a single twisted pair. You can get useful power through all of the wires (4 positive, 4 negative). Since the Fathom-X is based on an Ethernet over powerlines module, we can send DC power through the tether using all of the wires and use those same wires for Ethernet data.
As far as voltage goes, you can use a voltage drop calculator to estimate how much power you'll get at the end of the tether. The maximum power output is achieved when the voltage drop is exactly 50% of the input voltage at the topside. For example, if you send 4 amps at 48V and the voltage drops to 24V at the end of the tether, the ROV will receive 24*4 = 96 Watts of power. That's enough to keep things running and do some gentle maneuvering, but not enough to do anything substantial.
You can use much higher voltages, but I'd caution you against the safety issues with that. Most commercial ROVs use high voltages such as 360 VDC but they also include line insulation monitors (LIM) that protect against current leaking into the water. LIMs are not cheap.
Our approach will be as safe as possible while increasing the ROVs endurance to nearly unlimited. It's still in the early stages so that could all fall through still.