On a recent river rafting trip when I tried to charge my phone I got a new error message I hadn’t seen before: “Liquid has been detected in the Lightning connector.”
I had been doing some canyoneering that day, which involved doing some swimming through deep pools, so in a way it’s not surprising. However, I’ve swam with the phone plenty of times before and not gotten this message. Interesting that it popped up now.
An iPhone’s water resistance degrades over time. Perhaps it’s time for a waterproof pouch or at least a Ziploc bag.
I don’t think this indicates degradation in the waterproofing. Just that some water accumulated in the port (outside the seals) and it needed to dry out before charging.
In any case, for the way I use the iPhone while I’m on the river a waterproof pouch isn’t really practical. I’m willing to run the risk of damage.
When that message appeared on my new iPhone I Googled for advice. happened i think when rain drops got in when in my shirt pocket and the connector was upward facing.
Apple’s documentation was specific not to try to use a hair drier, compressed air, cotton ear buds, etc. And they said to wait till dry until plugging in to electricity.
I understand. I’ve never gone into the water with an iPhone but I’ve been soaked to the bone by the occasional thunderstorm. 
I’ve drowned a couple pair of earpods but the ziplock bag I usually carry for my phone has yet to fail me.
“Splash, water, and dust resistance are not permanent conditions and resistance might decrease as a result of normal wear.”
About splash, water, and dust resistance of iPhone 7 and later - Apple Support
I expect putting the wire into a wet port is an electrocution risk rather than a waterproofing issue.
Others have said the same, but just drying the port should do the trick!