Hello Marcus. Normally, a power supply is sold as a system, so that you purchase both the submersible vehicle power unit (that replaces the battery housing) and a surface power unit that supplies the higher voltage and also includes safety measures like earth leakage (GFCI), Line Isolation Monitor (LIM) and fuses. It can be very dangerous to provide power directly from a generator, unless it includes appropriate safety measures.
To answer the power question, in typically use 15-20A at 15VDC on the BlueROV2 heavy (8 thruster), but if running at 100% gain and pushing very hard, it can draw 60A or more. Our estimates are: 15-200W slow flying, no current, 200-400W normal flying, low current, 400-600W fast flying or strong currents, 1kW maximum for very fast flying or very strong currents. We find our 500W supply is good for most customers, but a few may want to look at the 1kW surface power supply from Outland Technology (available on BlueRobotics.com). Ours or theirs will both run on 115V or 230VAC and have appropriate safeties. We run 22AWG power conductors in our tether (2 total) and a bare earth wire for safety and that would work fine at 400m (about 20% voltage drop with ROV power 500W at 15VDC).
For the voltage drop, the following calculator may be helpful: https://www.calculator.net/voltage-drop-calculator.html
If Gigabit Ethernet is required at 400m, then yes, fiber is your only option, however, even many professional cameras will just record locally and only send HD or even SD video to the surface. I have attached a datasheet for our A/C power supply system. You can email Ramon Gomez Ramon@PoseidonROV.com in Spain or Jeff Conger Jeff@Blue-Linked.com in the US for more details.AC Surface Power Unit rev3.pdf (269.5 KB)