Hey Harold,
No, it’ll be fixed mounted. Haven’t had a chance to snoop the ethernet. Right now I’m focused on learning whatever this system is capable of doing and bugging Rusty to make some new components for my next ROV build. 
Ok, I will take the bait … what new components are you looking at? Inquiring minds want to know!
The thread lives on.
If you really want to go low cost, this paper which is freely available off the internet, is the is the definitive start;
UNIVERSITY OF CALIFORNIA, SAN DIEGO
Design of a Low-cost Underwater Acoustic Modem for Short-Range
Sensor Networking Applications
A thesis submitted in partial satisfaction of the
requirements for the degree
Doctor of Philosophy
in
Computer Science and Engineering
by
Bridget Benson
There are some practical forum threads over at OpenROV
Accoustic Modem != Sonar
Saw the thread over at OpenROV. It’ll be interesting to see how it progresses.
Without chasing down this document … is it about a SONAR system or is it about an acoustic “modem”. Been there played with the modem thing before. We were trying to control RC submarines so we could get real crazy on all of those skimmer models floating on the surface 
The main problem that I ran into was power and because we were in a lake different types of reverberation and absorption issues were a pain in the rear.
Anything that requires you to bang sound out of a transducer is going to require power. Water has this nasty attenuation effect depending on the frequencies etc.
Never mind … just read the article. So what are others saying about this “modem”
Here’s the Benson thesis for those who don’t already have a copy.
benson-2010 (3.17 MB)
Shouldn’t it be SODAR ?
Weird … I snagged a different document that was talking about underwater modems. Guess there are a few. I used the title in the original past about it.
Here’s the link to her website at Cal Poly: http://www.calpoly.edu/~bbenson/index.html
You’ll find all her research and publications there.
Here’s the publication that resulted from her PhD thesis, which may be the document that you were referring to, Harold?
oceans10-low_cost_modem (1.39 MB)
My old timers must be acting up. I just snagged the title and did a google search and here is yet another document that involves an acoustic modem. This is what I originally found.
The PDF is to large for me to post here and “Benson” is listed at the end as one of the reference documents.
When does remote control become a modem ?
Remote control can be performed using a modem. The modem is just a communications device period … no matter if you are just sending “data” or if you are sending “data” that is used to perform a task once the receiving unit digest it.
Frankly the control rate that I have seen in the various articles is pretty darn slow. Now granted I remember the days of 110 baud with half duplex because I owned and used a modem that slow. Frankly at one of the maximum speeds that I saw of 19,200 bps you could do some decent stuff but you would have to have it in both directions.
I don’t know what this acoustic modem would be used for other than just transmitting data that does not require real time action … more of a data logger application. Trying to move an ROV around would get real dicey real quick because you need to have feedback from the unit to know what the heck is going on. Forget video or anything else because you only have enough bandwidth to send commands and then the ROV would have to have some power output to respond … it doesn’t have to do much to just receive, but if it is going to be a two way street, then the power monster is going to show up pretty soon.
There is really no need for an acoustic modem on an ROV as it already has communication and control capability via the tether. AUVs are a different story. If you want to communicate with and/or control them, then an acoustic modem is essential (or one of the newer optical modems). The trend is towards “swarms” of coordinated AUVs operating in concert, which requires communication among the AUVs and possibly communication with an ROV and/or surface platform as well. It is also possible for vehicles to act as “relays” to extend the range of communication.
I presume that the optical links use blue or UV LED’s and detectors, or are these not available with sufficient power ?
IR, obviously, gets attenuated more, blue/UV are lower power to start with, but may be able to handle higher data rates … not that theoretical data rates would be a limitation anyway !
S.
Here are some links to learn more about undersea optical modems:
http://www.lumasys.com/index.html
http://www.aquatecgroup.com/index.php/underwater-communication-sp-17953