If it works I'll tell you,but I think it won't be before september-october.
I just saw the video of the OpenROV that MichaelMeissner linked (thank you!), it's a very interesting project. They obviously use battery to supply power to the ROV, I think I'll be better using rechargeable batteries too. Just need to be shure they supply enough current for the motor I'm using and all the other electronics.
For the communication they use a Ethernet to Single Twisted Pair Adapter.
100m is a long way down. It is going to be very dark, you are going to need power for lights. As was mentioned cable weight and management will be a problem. The pressure at 100m will 10 atmospheres above the surface pressure. Are you going to pressurise the interior of the ROV or are you hoping to keep a watertight seal against 10 atmospheres? How will you connect the cable to the ROV preventing water entering either the ROV or the cable sheaths?You should try to experiment in a swimming pool first. In the first 10m of your dive the pressure will double halving the volume of any compressible spaces.