EDIT: Anyway, it wouldn't be a problem if it did fall in the water, they're sitting in a nice PVC pool, no-where for the electricity to go, except for back into the extension lead, causing a short. No problem...
What if one put a foot over the edge to get out? I suppose the easiest way out for the electricity would still be through the wire - it would only zap someone if the only way out was through a person or the water (in which case it would choose the person).
There's obviously ways that you could still get a shock whilst in that situation, i'm in no way saying it's safe , just it's highly unlikely to cause anyone a problem.
It's still stupid though