Is it actually dangerous for fish that are not right next to the surface? After all, salt water is a fairly good conductor (low voltage drop per distance) and fish are small (low distance), resulting in a low voltage difference across their body.
If the fish itself is as conductive as the water, then I assume nothing at all happens.
So there is going to be a radius in which there are going to be decreasing levels of danger. Think like a stack of progressively small and smaller bowls. The top of the bowl is the top water and as you go from smallest to largest. The energy is going to dissipate. I think at an exponential rate if I am remembering correctly.
It is not the resistance of a body that causes electricity to kill. All animals including humans are pretty good conductors. Remember blood basically just slightly salty water. Electricity mostly kills by stopping the heart. Heart muscle cells are very sensitive to electricity. Death occurs the millivolt range
There can only be an effective voltage gradient when the conductivity is different. Hence standing on one leg you do not get hurt from lightning close by (only direct hit). But I have no idea about the conductivity of fish...