Actually, I think that's backwards. Saltwater IS more conductive than fresh, this is true, but that means that electricity will travel farther in saltwater. So you are at more risk in saltwater than freshwater. It has to do with the way salt breaks down in water, the chlorine and sodium...something something something...I just remember having to ad salt to water in science class for the lightbulb to light up.
The issue is NOT conductivity in the way you think it is.
As a point of issue, go look at the power lines outside. You see birds on them, right? They don't get fried, right?
The reason is NOT that they're not grounded. Its because the gradient (difference in potential) between their two feet is close enough to zero that they do not get zapped through their legs (which WOULD fry them.)
The same thing is going on here, and its the RESISTANCE that makes it happen - or not.
It is the potential
difference between any two points of your body that causes current to flow. You can be at a potential of 1 million volts, and all is well, so long as ALL OF YOU is at the same potential. Provided that there is no point of your body that is able to become at a lower potential charge, no current will flow and you will not be zapped. This is how they do the Van-de-graff demonstrations where your hair stands up on end - you are at a voltage of hundreds of thousands of volts, or even more - but ALL OF YOU is - so your hair stands up trying to "seek" a lower potential thing through which it can discharge. (Fortunately the current available is vanishingly small, otherwise if you managed to complete that circuit you'd be a human lightning conductor!)
With a LOW resistance the gradient (difference in voltage across two points) is lower; the field dissipates into the water more quickly. As a consequence, the "danger zone" is much smaller; you have to be closer to the energy source to be at risk.
With a HIGH resistance the gradient is HIGHER. The field dissipates into the water more slowly.
The problem is that the field strength DIFFERENCE between your arms, or between your arm and leg, or your head and foot, is what matters. The absolute field strength in the water means nothing its the DIFFERENCE that nails you, and the relative difference in resistance between your body (relatively low) and the water (relatively low in salt water, relatively high in fresh.)
If that field strength difference is high enough, and your body resistance is low enough compared to the surrounding water, you become the conductor between the higher and lower field strengths. The result is that you fry; more accurately, the electricity either locks your muscles (including your diaphram!) and you drown or it disrupts your heart's electrical impulses, and you suffer cardiac arrest. Either way you're toast.
Drop only the hot side of an electrical cord into both fresh and salt water. Then, being EXTREMELY CAREFUL not to become the "bridge", measure the voltage between ground and the water at various radii from the current source.
You will find something amazing - in fresh water the voltage to ground a good distance from the current source is quite high, while at the same distance in salt water it is a small fraction of the fresh water value - that is, if the salt water electrode drop doesn't trip the breaker before you can do the measurement.
People get object lessons in this every year in fresh water marinas. Every year some idiot uses a cheap car battery charger on their boat to charge batteries in a fresh-water marina. The problem is that many of these have a hot chassis (no transformer), and the 12V negative is connected to (line) ground. Now if the outlet is wired backwards, or you use a non-polarized plug, there is a risk of 120V being applied to the grounded side of the boat's electrical system, and through it to the water. The high resistance of the fresh water prevents enough current from flowing to trip the dock breaker, and nobody is the wiser that the water has become energized.
If you get in the water to swim (or dive to clean the boat!) you suddenly become a path for the relatively high field-strength differential to pass through, and your body has a lower resistance through it than the water surrounding you. You get electrocuted. This can happen even if you're 20' or more from the underwater metals, in some cases (quite clean fresh water, with realtively low ion count) it can happen at 100' or more!
Every year people die this way.
In salt water the field strength dissipates FAR more quickly; you have to get within a foot or two of the energized metals before you are at risk. While it
can happen in a salt-water marina, it almost never does.
The same situation applies to lightning. A hit in salt water is not a big deal, unless it hits YOU (or damn close to it.) A hit in fresh water is another matter, as the "danger distance" is MUCH further away. Given that the voltage and current levels involved in a lightning strike are in the "way outrageous" category, a larger danger radius bothes me!
This, by the way, is why I am of the opinion that boats in fresh-water marinas in particular (and in all marinas in general) should be REQUIRED to have isolation transformers - with them a leak from the on-board systems cannot leak to energizing the water, as there is no path back to the other side of the utility feed.
