We do quite a bit of this kind of stuff, data + video coming one way, control signals going the other way. Although never underwater. So I've built quite a few custom interfaces for this kind of thing. There is RC software out there that might well do much of what you want, but we've pretty much always gone to customised interfaces in the end - a large part of that is because we're often doing stuff for public display so we want it to look nice, but to be honest the hassle involved in creating your own software that works just so is usually less than fudging the existing software to almost work well enough.
From your description, I'm slightly confused as to where the computer is - is there a computer on the ROV, or only at the end of the tether?
Whatever, presumably given there is an IP camera, there is ethernet or equivalent in the tether cable to get that back?
Personally I'd use an arduino or similar microprocessor board to get the sensors, and talk to the motor controllers, rudder servos etc.
Arduinos are just a really easy way to interface PCs to hardware with enough program space for a tiny bit of intelligence in case of the connection to the PC failing (eg. assuming there is an accelerometer + gyro, you could make it surface automatically on loss of network connection).
Then I'd use something like Processing to do the display & control - you can easily hack together a bunch of inputs into graphs / text, and overlay them on a video.
There is a little bit of ability to program required in using both arduino and processing, and you need at least a vague knowledge of how to hook up servos / motors etc, but a)the programming is pretty easy - they're both aimed at artists primarily, who don't typically have a programming background, and b)I imagine anyone trying this kind of project at least knows how to control a motor.
How you send the data across depends somewhat on what I said above - ie. whether you have a computer on board.
If you do, then I'd use a USB cable connection to the computer, then use processing (or python if you happen to know that), to make a simple server program that just reads data from the arduino and sends it over the network.
If you don't have a computer actually on the ROV, then I'd buy the arduino with ethernet built in, and just send the data straight up your cable to your processing script (you'd need some kind of hub/switch in the ROV if you're using the cable for the IP camera also).
I wouldn't recommend a wireless telemetry unit as suggested above, as I think they typically don't work underwater by the way.
In terms of timestamping and storing the data - personally I'd do one of two things - either use processing's "sqlite" database to bung it into a simple database, or just write it out to a text file as comma separated values. The pain however, is syncing it to your camera, especially when your camera has it's own independent clock, and being a webcam / IP camera, will probably be pretty poor in terms of sync etc. Sync is hard.
Oh, make sure that wherever you timestamp the data, it is the same place. Ideally wherever you are saving your video data, because then at least the start time of the video should be in sync with your data.
Oh and assuming your computer is fast enough to run it fraps is brilliant for getting a quick recorded video of what has happened onscreen during an event for later replay, we pretty much always run that as a backup in case for some reason data storage fails or something.
