First, what a cool site, I've had plan for a project like this for years I just never seem to get around to it, but back to the video latency.
Although it would mean shifting away from webcams and going with a standard CCTV camera there is a way to transmit video feeds lag free and on the cheap:
Passive Video Balun's
These things sell for around $10 each (you need one for each end) and they allow you to send video signals over twisted pair cable, like Cat3, 5, or 6.
It would be possible to utilize the other pairs for powering the camera or communications. Advantages of switching to a CCTV camera are lens selection and low light sensitivity. You might even be able to cram a controllable zoom lens into one of the ROV's if you wanted, although I'm unsure of how the pressure would effect operation.
Hopefully this isn't a repost, and please e-mail me if I can provide more info,