@tasman Consider this a supplement to the excellent answer by @hellomacos . First stop continuing to try to charge, sync or otherwise use your phone. This potentially can make a bad situation worse.
Salt Water will get your components to corrode very severely. The very first step I would take is to disassemble your phone. Use iPhone X Repair to disassemble your phone. Take a careful and close look at all components and connectors. Truly assess the damage. Look for major corrosion, evaluate the pins in the connectors etc,. Take a look at all connectors as well as cable ends. After that I would suggest that you clean it a couple of times in sterile water, not drinking water (that might have helped right after it got submerged in the salt water). Use a soft brush and clean it very, very thouroughly with the sterile water. With the amount of corrosion that I envision, you will have to remove all the EMI shields. Once you got the gross contamination cleaned, use 90%+ isopropyl alcohol and clean your parts some more. For a proper cleaning, use this guide. It was written for a Apple iPhone 3G but it is still pertinent to your phone as well. I can not stress the importance of a good cleaning enough, so do it over and over while replacing the alcohol after each cleaning. Do not use compressed air for the cleaning since that can drive liquid as well as corrosion particles into areas that will cause trouble later on. Do not use drinking water, if you need to use water, use sterile water. Also, you do not need to leave it to dry for a day. Isopropyl alcohol in the higher concentration will evaporate quickly and dissipate the water. The importance here is the cleaning. The best cleaning would be by ultrasonic cleaner but if not available, this will help. Do not forget to replace the battery. This is a must and not an option. It will fail if you do not take care of it now. Keep in mind that water damage is the hardest to troubleshoot and to repair. In your case it is possible that a new display will resolve all of your issues Hope this helps, good luck.