You ask about diving on a boat in fresh water. Is it less dangerous to dive in salt water?
Technically, yes, but don't bet your life on it. It has to do with the voltage gradient in water. In fresh, which doesn't conduct all that well, the gradient is higher across a given distance than in salt water, and the human body is more conductive than fresh water. Stretching out your arms to span a big gradient results in you conducting enough current to cause you to tense and stop breathing and moving. Salt water, which conducts much better, will have a lower voltage difference between the same two points, so you're conducting less current all things being equal.
You don't have to stretch your arms out, by the way; I was just using it for illustration purposes. But if you feel funny in the water next to a boat or dock, the advice is to pull it all in to make yourself small (lower voltage difference extremities within a given gradient), and if possible, swim away from the boat, dock, or whatever could possibly be causing the leakage. But if you're in real trouble, you won't be able to do anything.
I've always heard there have been no reported cases of ESD in salt water, but it's often misunderstood and not considered in investigations so who knows. And I don't believe a medical examiner can tell the difference between ESD and 'regular' drowning, so it's probably way underreported no matter what the salinity is.
But at least it's getting publicized now. I told my now 85 YO FIL about it long ago, and he told me it was 'common practice' many decades ago, at least among his boating buddies, to snip the ground connection to the power pedestal to reduce galvanic corrosion on your boat. Any ground fault would, of course, energize the water through whatever is bonded to the ground bus.
