Salt H2O damage, multiple issues

skendrot, the importance is not how long or how it got exposed, but where it got exposed. The major issue you will run into is corrosion damage. Salt Water will get your components to corrode very severely. The very first thing I would do is to disassemble it by following this guide. Take a careful and close look at all components and connectors.Look for 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. Use a soft brush and clean it very, very thouroughly with the sterile water. With any chance of corrosion, you will most certainly have to remove all the EMI shields as well. 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. Isopropyl alcohol in the higher concentration will evaporate quickly and dissipate the water. The importance here is the cleaning. Yes, you did well to replace the battery, but I would also prepare to replace the dock connector. Once it is properly cleaned, reassemble and re-evaluate. The WiFi could be an IC issue and may require a reflow. Not uncommon after water damage. Replace any parts one at a time so as to not spend unnecessary money and time and effort, as well as confusing the issue. Even with all this, there is still a chance that your phone will not work, but I definitely would give it a try. Hope this helps, good luck.