Or a combination of water & a vast amount of electricity, I know which I'm siding with. Tha knows, metal hand rails throughout the entire length of the bridge, buzz buzz.
I may not know too much about electrics but it strikes me that if the earthing in the electric box is so shoddy that a fault would result in the handrails being live, then they have bigger problems to fix than a bit of water.
I'm no engineer, but for £12 million, can't they design and build it right in the first place? It's not as if it's been a rushed job either.