disassemble iphone and clean parts and connectors with isopropyl alcohol and a soft brush. residue from water is either white or green. allow to dry, check and clean again if necessary. let it dry for a few hours to be safe, then reassemble. You can submerge the motherboard in the iso for a short time, give it a gentle jiggle to make sure iso gets everywhere and to remove the residue.
reassemble, charge on the mains charger, and let us know how you get on.
the battery is the most likely part to need replacement which is why I suggest mains instead of pc etc so you can test phone for what does and doesn't work.
If it happens to get into the sea with the water , I believe it can be Fixed. There is no electronic device that cant be fixed. Neither Apple would say it can be fix cuz they would want to make money and not encourage people to fix their phones by themselves (Technic of Marketing). Read guides. Use the forums..If you love your device you will do it. Opening the phone , cleaning with some alcohol doesnt makes me so sure as it can damage some parts - Cant be sure though - I'd try replacing battery after super cleaning the phone with some alcohol or else the old battery might re-damage the parts and fully destroy it. Water is not a matter to think about it. Salt water is a serious business but there is still chance to get it fixed.