Watch doesn’t turn on

I have an Apple Watch Series 6 - long story short I went swimming with it on and had the water lock feature turned on. It then died due to low battery and now when I try turn the device on it’ll show the Apple logo and then switch off instantly. I went to the Apple Store and even though they didn’t know what the issue was they wanted to charge me £280 to fix it. Can someone help/advise me on what to do?