How does one get electrocuted in a lake?

This question arose because my son was asked to leave the pool during a swim lesson because one of the lifeguards saw lightning. He was on the inside part of an indoor/outdoor pool. They had to stay out for 20 minutes.
I understand how it happens on land, you become the path for moving electrons which is very bad for you. But on water, how do you become the path? I’ll assume electrons would simply take a path around you to get to the ground. This is the response I got to a previous question about lightning. If you are sweaty, the lightning may travel along your skin and then into the ground saving you from serious injury.
How close does the lightning have to be to kill you? A lightning strike on one side of a lake couldn’t kill someone on the other side (unless you have a very small lake). I suppose you might be the tallest thing in the water and you get struck directly but that doesn’t explain why the pool had to be evacuated if my son was inside a building even though the water was connected to the outside.
Any ideas?

It’s a question of potential difference. Say lightning strikes a body of water. Picture a series of concentric circles, with the highest potential in the middle, at the strike, with declining potential as you travel away from the center point. This will rapidly dissipate as the current goes to ground, but exists for a short time.

Now picture you, some place in the circle. If your feet, for example, are at one potential, and your head is at another, there would be a current flow. How much flow will depend on the potential difference, and the resistance offered, but if the potential difference is high enough it could be fatal.

I heard a safety lecture once by someone who said the same thing applied to downed power lines, and when leaving one you should always take baby steps to avoid setting up this potential difference.

People often say that electricity takes the path of least resistance. This is only approximately true: Electricity mostly takes the path of least resistance. If it has two paths of about equal resistance, it’ll split up about equally between the two paths.

In many regards, it’s a good approximation to treat the human body as just a big blob of water. In particular, the conductivity of a human body is similar to the conductivity of water. Skin offers some resistance, but air is a much better resister than skin, and that lightning bolt has just zapped through a whole mile of air without a problem. So about as much electricity will travel through your body as through any other region of water the same size. Admittedly, you won’t get all of the lightning strike, but you’ll still get a fair chunk of it, and a fair chunk of a lightning strike is not anything I want to mess around with.

Only about 5% of lightning injuries come from direct strikes. Many more injuries are the result of the current spreading through the surrounding ground (or water) as it dissipates.

I don’t think that would save you to any significant amount. Given the high temperature of a lightning bolt, the miniscule amount of sweat on your skin would be vaporized away in an instant.

Electricity kills people in basically two different ways. The first is by screwing up your heartbeat. If you get a shock to your chest at exactly the right time, your heart tends to get its beat screwed up, and once it’s screwed up, it tends to stay that way and won’t fix itself. At that point the best you can hope for is that someone has a portable defib unit somewhere nearby. The second way electricity kills you is by simply cooking you to death.

Cooking you to death requires a fair amount of energy. A lightning bolt has more than enough energy to do the trick, though, and most lightning injuries are actually burn injuries. People lose legs and arms in lightning strike due to the limbs being literally cooked to death. Recovery from lightning strikes is usally a long, painful experience due to the burns.

Screwing up your heartbeat takes a surprisingly small amount of current. Currents as small as 50 or 100 milliamps (0.05 to 0.10 amps) are capable of stopping your heart. To put it in perspective, your household current in a typical outlet (in the US) trips the breaker at 15 amps in a fault, and lightning is usually a current flow up in the hundreds of thousands of amps range. When lightning hits the ground, the energy is going to be spread out in all directions, and it is mostly going to take the path of least resistance, through dirt, plants, swimming pools, water pipes, etc. If you are in the pool at the time, you only need to get a teeny tiny fraction of the energy from the lightning bolt (like about a millionth) to potentially have a fatal shock from a lightning bolt that didn’t even hit you directly. A lightning bolt a fair distance away from you isn’t guaranteed to kill you, but it isn’t guaranteed not to either. It’s enough of a risk though that if I see lightning, I’m getting the f*** out of the pool!