My wife inadvertently dropped her iPhone 6S into the ocean while powered on. She immediately removed it from the water and gave it to me. After unsuccessfully trying to power it down, I dried the outside and put it in my pocket. I did not have any tools with me and could not open it up - when we returned from the beach, we resorted to rice (used as a desiccant) and waiting. The next morning, we were able to power it off but the screen was behaving weirdly - white lines, with an intermittent grey and blue screen. Two days later, we tried to turn it on but it only looped with the apple logo and white lines. After a visit to the apple store, apple told us it was unrecoverable and offered to sell a new one for $299 with trading in the damaged phone. However, my wife had pictures on the phone that were not synchronizing to the icloud and desperately wanted them.
Upon returning home to New York, I opened the phone and saw damage to the charge-port connector slot. I ordered a new lightening connector, loudspeaker, and battery. I also removed the logic board (which is not necessary when replacing only the lightening connector) and saw significant "frying" on the underside of the processor closest to the display connectors on the top of the logic board. I removed the logic board and placed it in 91% isopropyl alcohol for 5 hours. I periodically swished it around. At the end of the 5 hours, I repeatedly and gently brushed the underside where it appeared burned with a small soft brush (used the one that comes with the ifixit repair kit). I let it dry overnight and the next morning replaced the logic board. I started itunes on my PC and connected the iphone to do a restore. It successfully worked and preserved all content on the phone. I then charged the battery fully and confirmed that there was no overheating around the damaged logic board area. This area previously got very hot when I attempted to start the iphone.
A cursory inspection of the rest of the phone showed damage to the pins on the earpiece speaker and the sensor cable contacts (some looked like half-moons) and one spring was 3/4ths missing - water damage was seen here also - probably through the earpiece slit. I ordered an earpiece speaker and front- camera sensor cable.
In summary, there was water damage visible in 3 areas:
1) around the earpiece speaker inside the phone
2) above the sim card slit under the logic board (most significant here)
3) one pin on the lightning connector was damaged (by sand), However surprisingly, little
water got into the bottom of the phone
The two most vulnerable areas of the phone to water were the earpiece and sim-card tray entry points.
By the way, I could not figure out where the tiny rubber cushion that fell off the logic board was supposed to go - I reinstalled the board without it - no issues.