There is no direct evidence that Tim Cook actually said this in quotes. This was actually an employee (In Germany) stating they actually
overheard or was privately told Tim Cook Mentioned this about the first generation Apple Watch when it launched in 2015, that he showered with it on. Although I'm inclined to believe that he did say this, it was never actually confirmed Concretely.
However, the first generation Apple Watch does have an IPX7 splash resistance rating. Although many have showered with it, experiencing no problems using it in the shower, which Apple likely undermined it's rating.
https://support.apple.com/en-us/HT205000
https://9to5mac.com/2015/02/25/apple-watch-water-apple-pay-europe/
https://www.technobuffalo.com/2015/02/25/apple-watch-water-resistance-shower/
https://amp.businessinsider.com/tim-cook-apparently-showers-with-his-apple-watch-2015-2
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Except Apple directly states not to and it wouldn't be worth risking warranty or costly repairs:
Direct from Apples website:
https://support.apple.com/en-us/HT205000
"
We recommend not exposing Apple Watch to soaps, shampoos, conditioners, lotions, and perfumes as they can negatively affect water seals and acoustic membranes."