Looking at the tether decision tree, I really hate the idea of a electrical connection underwater. So after a little brain storming a I designed a Cape for the Beaglebone that transmits and receives over a pair of TOSlink cables to a USB Transceiver.
Based on the data sheets for the for the AM3359(Beaglebone CPU) the UARTs of the beaglebone support data rates of up to ~3 Mbit/s. The USB Transceiver supports data rates of up to 6Mbits/s. The system will transfer data at 3Mbit/s which is more than enough for a control signals and video. The actual tether is currently 2x 2.2mm OD POF cables.
I have a few developer boards, usb transceivers, TOSlink jacks for sale. If there is interest I will do a production run.
Also I understand that the AM3359 supports s/pdif as an IO, this is not an implementation of s/pdif
BeagleBone Bark Cape
Cape and Transceiver Connected by Coiled FOP Cable
Update 1: Let me see if I can address some of these questions.
Why I chose TOSLink? There are fiberoptic standards out there with both incredible range and data rates. But the cost of these systems are very high and in almost every imaginable use case, overkill. The cost of TOSLink is several orders of magnitude lower. This is because of it market penetration in digital home audio. Additionally, TOSLink connector are designed to accept 3.3v and 5v TTL signals.
USB Transceiver model - This is something I designed and manufactured for this purpose. The Whistle USB Transceiver is USB to TTL converter IC connected to a TOSLink Transmitter and a TOSLink Receiver. Inside of a case that is I printed on a Thing-o-matic. More information on the design is available in its git repository
Range and Attenuation
Here is where I am still operating theory here as I just manufactured the boards this past week and haven't tested them with long cables yet. The industry has seemed to settle on 30m as being the longest usable TOSlink cable so we will see what maximum length is in the coming weeks. If the max length is 30M then I already have a repeater system on the drawing board.
Short cables are cheap, super cheap. Like $7 for two 25' cables. The longest commercially available cable is ~30M (100FT) and costs about $60 for two.
The cost of a repeated 100m cable could be as high as $200 in the early stages of development.