One thing people really don't understand about electricity and water: a shock that would be trivial anywhere else is fatal when water is around.
You're hit with a triple whammy. First, ordinary water and wet skin conduct electricity well, meaning a shock you barely felt before, now has much worse effect.
"Water doesn't conduct electricity" only true for artificially demineralized water; water does not like to be demineralized. Demin is corrosive and will attack anything it can leach minerals out of. The city's water source is not demin, and they don't entirely demin it; it would eat the pipes if they did.
Second, when water is around, a stun is as good as a kill. People in electrical drownings weren't electrocuted to death. They just lost control of their muscles and fell down into the water. They did not raise their nose out of the water because they were unable; either their muscles would not do what their brain commanded, or they were laid unconscious by the fall (or the shock). See how this becomes a domino effect: your legs malfunction, your head cracks the ground, which knocks you unconscious, and then, you drown.
And third, the same thing happens to the people who try to rescue you. In one famous drowning in a fountain, one girl fell, another girl went in to rescue her, the other two went in to help them, and all four died.
That's how it goes; the third or fourth cohort of rescuers recognizes they're looking at a victim and a rescuer who has become a victim, and does not repeat the mistake.
Of course it's made worse by the first factor; people who have been bit in the past and live to tell the tale, start thinking they're immune to electricity, and don't realize they've just been lucky so far due to being in the dry.
There is no way to insulate an extension cord connection. If it gets wet, it will leak current.
"Oh, but my house is protected by the bog-standard Euro 30ma RCD! I'm fine!"
Nope. Personnel protection is not the purpose of Euro style 30ma RCDs. 30ma will stun pretty much everybody, and will directly kill a more vulnerable person. That is why North America requires 6ma GFCIs where the objective is personnel protection (e.g. kitchens/bathrooms/pool areas).
Now, your hot tub's electrical cord may have a lumpy 6ma GFCI/RCD on its electrical plug. Obviously, the extension cord connection will be before that, so, not protected.