I simply tried to moderately blow some air into the USB port and there's a notable difference. Also removed the sim cap and sucked on the port. Air was clearly passing through.
I'm curious if it's just my phone with this issue in which case I'll replace it, or if it's a design fault, in which case you shouldn't submerge it.
In the screenshots you'll see the pressure difference. So please test this by accessing the service menu *#*#service#*#*, > tests > pressure sensor and simply blow some air into it.
And for the giggles and ranting, there's the sony talk thread :P https://talk.sonymobile.com/t5/Xperi...ght/true#M2652