Thanks Douglas, tapping the data off with a coil was the sort of thing I was imagining, great answer! But I'd need to know the "frequency" of the data signal to calculate the RLC network that results - it will basically look like an oscillator?
150V would be great but I'm just too safety conscious :) 48V is the highest I'd feel safe using at sea (salt water is both conductive and corrosive!) and it can easily be produced from 4x12V batteries. I will not use remote sensing, but allow the voltage to drop. The DC/DC converter I'm looking at keeps going down to 36V and I'm dimensioning to have that arriving at the ROV end even at full draw, which I've estimated to be 8A with a healthy margin (20%) - the capacitors are there just to provide that little extra, and because we can :)
And thanks David, I'll be documenting my design and sharing the details - once I have an actual design that is! Currently working on the thrusters (which also differ from the norm!) and the power supply side, including the tether (for which I plan to use speaker cable for it's thin insulation, and a strength member of some sort). Only when I have a grip on these two will I progress to hull design. Power and thrust dictates everything else! Needless to say, I am not going for the OpenROV hull, though I'm hoping to make use of a lot of your software and some of your hardware, like the IMU for example. So the control side will be the same as the OpenROV, but pretty much everything else will be different :)
Edit: Re. the hull - yes, a tubular, or even spherical, hull would of course withstand much greater pressure. The idea to use a die-cast aluminium box (or boxes even, if my connector design proves viable) springs from several thoughts: 1) a lot of people are successfully using OtterBox enclosures, even at 30m depth (my target) 2) aluminium solves the heat dissipation issues, and thereby hopefully condensation problems as well 3) it would be much easier to work with, both in terms of mounting stuff inside and in making pressure tight connections to the outside 4) it's cheap! The current plan is to get one of the Gainta enclosures (they look quite sturdy!) and beef up the seal with an additional flat rubber gasket, make up two dummy connectors with a loop between them, head out to sea and lower it to progressive depths (10-50m) to check if it remains dry inside. This won't happen until sometime this spring though!