For someone who grew up at the beach, I find Letten a complete joke. There are far better places to go swimming in Zürich. Go to Greifensee or to the lake at Rapperswil, far far better.
Letten is like a canal bordered by grey concrete walls on both sides and water flowing through it. Why in the world would locals like this place amazes me? It's just one of the places to be seen ... for locals only. When it gets busy the place is absolutely packed and you lie side by side with locals like in a can of sardines.
Also please be careful. There are no life guards and no one will save you. Remember that swimming in fresh water is far different from swimming in the sea, it's hard to stay afloat. I know of someone who drowned here and despite the cries for help, no one came to his rescue. He passed away...
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