I read the notes from your hackerspace meeting and I'm really excited to see that people are talking about this! Greg had a great reply which I think gets most of the info you need across, but I can also say that for OpenROV, I've been picturing a 100m tether length using very thin and low cost wires like this which are light enough to not need to worry about adding buoyancy compensation and are only a few cents per meter. I like being able to send both power and data over the tether (power can be sent at high voltage allowing lower current and thinner wire), however OpenROV can carry 8 "C" sized batteries, which at around 5000mAh each, would power the ROV for an hour or so.
It seems like you guys already explored a lot of the options for wireless, but certainly low frequencies can be used for low bandwidth communications as well as acoustic technologies. David and I are meeting with Pierros today, so perhaps some more thoughts will come together this afternoon.
I'm also very interested in using low cost acoustics for location finding- I wonder if it would be possible to build a a small, simple, acoustic beacon (maybe using a piezo element) that goes on the ROV that can be heard from a series of similarly simple underwater microphones which could trilaterate the ROVs placement in 3D. Just a thoght.
I'm excited about this discussion, and I look forward to seeing where it goes!