Running A Webcam 600ft underwater
USB over 600 feet is a no-go, at least directly. USB2 uses a 480 Mb/sec data rate, so DSL/ADSL, with a 9 Mb/sec, is not going to work. 100baseT Ethernet has a nominal max cable length of 100 m between routers/switches, so you'd need a box halfway down your cable. Plus, you need considerable extra cable length to keep the cosine law of tension from causing your camera to walk. For 600 feet, anchored at the seabed, I'd recommend counting on a 1000 foot cable.

I'd suggest your best bet for video transmission is fiber optic. Deepwater ROVs do this routinely, so it's clearly possible. By commercial standards your 600 foot depth is modest, so it's technically feasible, but starting from scratch as a complete noob to the field will make things difficult. You'll need a fiber connector which operates at 300 psi, and those are not, so far as I know, an off-the-shelf item.