Alex the major issue you will run into is corrosion damage. Salt Water will get your components to corrode very severely. The very first thing you want to do is to not plug your phone in, try to sync it or turn it on. After that disassemble it by following this guide. You must remove all EMI shields. After that 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 ocean;-). Use a soft brush and clean it very, very thouroughly with the sterile water. 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 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. 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. Also, I suggest to replace the lightning connector. There are no standard item besides the battery and the lightning connector that need to be replaced. Cleaning will only get you to the point of proper troubleshooting. Unless your phone is properly cleaned, there is no way to do that. Keep in mind that water damage is the hardest to troubleshoot and to repair. Hope this helps, good luck.