Hi,
I am working on an ROV. I use pressure and temperature sensors. And i have to control 4 brushlesss motor. andd video streaming. I will use max. 20-25 m. ethernet cable. Is it necessary something such as fathom-x? So sorry my English 
If you are using a low resolution analog video camera, then the Fathom S interface will work.
If you are using a high resolution USB or RPi video camera, then you will need the Fathom X interface and a RPi and any required support hardware.
You can use a normal ethernet cable. You do not need an interface board for a short length (< 100m) of ethernet cable.
Thank you.
Its a Small Task You want Need Much Stuff , Yes Jacob is right go with that thing.
Actually, Iām worried about video transfer. Because we need live video to move on. Then there will be no problem?