Excellent question, at sea level the atmosphere is pretty dense.
With that much kinetic energy, even a near miss is devastating, just from the sonic boom that follows in its wake. Accuracy is not a primary consideration, the range will be relatively limited due to the density of the atmosphere.
The sonic boom will have effects below the surface of the water as well. There will be a lot of marine life rolling up to the surface after such a discharge.
Now as to a potential source for this much electrical power I would suggest the following:
There is a technology called “Plasma arc trash reduction”, a process by which ALL forms of trash are reduced to their constituent atomic structure, then the heat generated by this process is used to drive electric power generation. The primary products of this process are “syngas”, a mixture of carbon monoxide and hydrogen, both of which are excellent fuels that may be used to drive the generation of electric power, and a silica slag which contains practically all other components of whatever went into the trash stream.
Once up and running, the operating temperature of the plasma torch is about 33,000 degrees F., about three times the temperature of the sun’s surface. The syngas generated is about 2,200 degrees F., and is passed over a heat exchanger to generate superheated steam (diverted to the generation of electricity), in the process of cooling it. Once cooled, the stream of hydrogen and carbon monoxide may be separated, yielding up pure hydrogen which may be used to power a fuel cell, or burned directly in the presence of oxygen to yield a very hot flame, which may be used to further produce power through the medium of superheated steam. Carbon monoxide itself is an excellent fuel which when combined with oxygen, forms carbon dioxide, a safe, NON-POLLUTING fraction of our atmosphere, and one that is vital for the photosynthesis of oxygen and carbohydrates in green growing plants. The carbon dioxide may also be captured, cooled and compressed into either liquid CO2, or allowed to become “dry ice”, an intensely cold and solid form of CO2, and an important industrial product.
The hydrogen, of course, when combined with oxygen, becomes water vapor.
Now, if this sounds like “perpetual motion”, keep in mind that the system, once charged up and functioning (by creating the plasma arc in the first place), it is possible to generate from three to six times the power necessary to maintain the arc, so long as the trash stream continues to be fed into the reactor chamber. The surplus electrical energy produced then goes into powering the rail gun, or converting any “syngas”, not consumed in generating additional electric power, into hydrocarbon fuel, among other byproducts.
Scoop up the floating debris on the ocean surface, divert all the existing waste into electric power, and reduce need for and dependence on fossil fuels. And not only the floating debris fields, marine life kill and the sewage sludge that is now dumped by ships at sea could go through this plasma arc, with the decomposed matter adding its bit to the “syngas”.
And we don’t even need either fission or fusion power to power this system.
After that, the drag doesn't pick back up to appreciable levels until it's reentering thick air again.
So the 100 nautical mile range is not far fetched.
And if for some reason your within line of sight... you're toast almost before you see the plasma flash as the projectile leaves the barrel.