The phone fell in the water for less than a second.
Even if I opened it, dried it, and waited before running it again, the screen got broken quickly (two stripes of dead pixels, the touching screen doing whatever it wants then black screen).
When I start the phone, I got the usual vibration saying the power is on, but the sceen stays off.
I tried to put another screen (wihch was faulty, still wich displays images) and I got the same result.
Is there any way to access to the internal storage via USB cable and boot mode, or whatever ?
I don’t see one, if booting the phone blindly into the OS (supposedly) doesn’t already give you this access, which shouldn’t happen if not deliberately prepared for in advance.
The /e/OS recovery, which should be on the phone, doesn’t do MTP (the file transfer capability exposing Internal Storage to a USB connection).
The alternative TWRP recovery can do MTP, but you would need to boot it on the Fairphone 3/3+, which is only possible with an unlocked bootloader (not the default situation, or even a common situation), and then TWRP would need user input to decrypt the encrypted Internal Storage.
Until anybody else comes up with anything more promising, your best chance should be to get the display module to work somehow, hoping the core module is not the issue.
You could try disassembling, drying and cleaning the modules again, or you could have a look whether you happen to have Fairphone Angels (community volunteers trying to help users) with Fairphone 3/3+ expertise and possibly parts in your vicinity who perhaps would be able to help … https://forum.fairphone.com/t/fairphone-community-map/26553