If it works I'll tell you,but I think it won't be before september-october.
A few things - cable related-wise:
1) Have you thought about how heavy 100 meters of cable will be - will your ROV have enough power to overcome the mass to move it?
2) Can you provide enough power down such a cable (taking into account resistance losses, etc) to move the entire mass of cable and ROV?
3) What about your cable management system? How are you going to pay out and pull back in that amount of cable (and keep it organized and untangled)?
4) Is the cable jacket certified to withstand the water pressure at that depth? What will you do to keep water out?
5) Most ethernet cable is made with solid-core wires - these don't stand up to repeated flexing. Multi-stranded wire ethernet cable does exist - but it ain't cheap. The problem is, you want something even more flexible than that ($$$$); such specialist cable does exist, but it makes the stranded stuff look inexpensive.
As far as keeping water out of the ROV itself:
I would look into using BLDC motors and an on-board battery pack; run the motors in the water - spray them with silicone lube before immersion, and do a complete tear-down and cleaning after the run. The battery pack should be sized to allow the ROV to move the mass of the tether/cable. Add floats to the tether to make it more bouyant; I can't tell you how to manage 100 meters of tether/cable, though. Some kind of winch and spool with slip-ring connections would be needed, most likely. On the interior of the ROV, you'll want to displace the air with oil (mineral oil would work well - it's non-conductive, so you can immerse your electronics in it, and it won't spoil).
That's about the limit of my knowledge of these things; most homebrew ROVs don't even get close to 100 meters in depth...