I've seen rumors that Apple started waterproofing phones after Chinese criminal groups started farming parts on AppleCare by dumping the mainboard into buckets of Shenzhen seawater to deny electronic serial number readout. Your logic board can't be so dead from normal use that not even its PMIC respond to commands if it's waterproof.

I've also had iPhone dying from gasket leaks, the circumferential double sided tape seal dries out after a while.