The Shannon-Hartley theorem couples Bandwidth and Signal to Noise ratio to how much information can be transferred over a medium.
Let's just assume that there is a certain Bandwidth available, it doesn't really matter what it is. Then the Signal to Noise ratio becomes the only important factor. Note how it is a ratio where Signal and Noise both have to be expressed as power.
A coaxial cable is a from of a transmission line where both ends of the line (cable) have to be terminated properly. Most Coaxial cables you will encounter in the real world have a characteristic impedance of 50 ohms. That 50 ohms determines the ratio of voltage and current. Also, if you know the power of a signal going through that cable you can calculate the voltage and current.
So the signal (or noise, noise is also just an unpredictable signal) going through the Coaxial cable is just voltage and current, same as with any other cable. The cable is also just a "medium" for transport of the signal. Ideally the cable doesn't care about the voltage, current or power level. In practice there will of course be limits to what a cable can handle but usually we can ignore these limitations.