Water Damage!!!

Apple do not advertise devices as waterproof. The term used is water resistant

This is classed as accidental damage and will not be covered under your warranty 

I'm sorry to bring you bad news however you can fully understand what water resistance actually means with the help article below.

About splash, water, and dust resistance of iPhone 7 and later - Apple Support

I had this same issue. We were down in the keys on family vacation and I took a few photos under the water at the reef. My phone fried a few hours later. Luckily I sent all the photos I took to my Facebook before it died because it wouldn't connect to WiFi to back up on the cloud. I have apple care so it was $99 dollars to replace it but i had to pay an $800 hold, which was very inconvenient. They should advertise differently being that not all customers can afford to pay for a new phone because of a simple mistake. I did find in fine print that it is not recommended to take photos under water but that was after the fact. Best of luck, I am not sure if the phone can be fixed elsewhere. I am currently thinking of switching to android after the week I have had with apple, nothing has been easy.

Using the words water, depth and time would suggest it to any unsuspecting owner!

You and I know what the real situation is but the marketing folk use IP6 test pressures and times to look as though it really means depth in water for a time period.

SIM tray for example is not part of the tested sealed volume.

They are NOT advertised as “waterproof,” they are water resistant.

on the website / the part before the small print

This is 11 but they follow the same pattern

Where exactly were you told you could use the phone underwater?

Nothing in that statement tells you to use the device underwater. Again, notice the the word “resistant,” not proof.