Hello everyone!
As the guy who sent it in, where I was initially suspecting the water to get into the device was through the place where the behind-your-neck cord meets the casing. I will be honest, I never figured the best way to take them off from my head and I sometimes got a weird angle on that cord, when I tried either to take off the swimming cap or to take the cord out from under the cap. Now I bought a newer model and it appears that Sony also acknowledged this area as the 'danger zone' and added a large rubber pieces:
EDIT, there is a microphone visible in the casing at 5:05 on the left (bottom part, near the buttons), for making calls via Bluetooth.
Also, 'shoutout' to the Sony Poland Service Centre, that claimed that there's nothing wrong with this device, 'It's just completely discharged'. Oh well
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Cheers!