I've read sharks see the cables as potential (ha!) prey due to slight EMF leakage. They chew on them damaging them. Not the fiber optic ones of course.

All trans-oceanic cables carry an electric current in addition to light in fiber(s). Electricity is what powers the optical amplifiers of which there needs to be one every hundred or so kilometers.

For short-ish cables (a couple hundred kilometers), driving the amplifiers via light (interestingly this can be done from both the data-sending and receiving end!) is possible, but not for trans-oceanic distances.

Yeah, the amplifiers need 10,000 volts. The way the cable is constructed is to have the fiber pairs inside a copper tube (there's some other stuff in between too). The voltage is carried by the tube. The ocean itself serves as the ground.

In general the power to the amplifiers is series connected and the ocean does not come into the question at all. If it did you would have huge issues with corrosion even ignoring the effects on the ocean itself. So, along the cable there is a wire that carries small DC current that goes between the amplifiers, each amplifier places zener diode along this wire and gets its power from it. At each landing station there is a current source (that is capable of developing significant open-circuit voltage) that powers this (this scheme is the reason why the cables are laid not only in loops, but in "loops-of-loops").