water damage and phone won't turn off

I doubt it. Salt water is one of the worst things you can subject any electronics to. Your only hope, and this is a long shot, is to soak it in distilled water. Be aware, the distilled water will most likely short the battery out anyway, if it is not already shorted out. The battery should really be removed & replaced before soaking the rest of the phone in distilled water. The idea here is that the distilled water will remove any residue from the salt water, then you dry everything out, replace the battery & hope for the best. Again, even doing this is a long shot.

Leave it in the ziplock bag for at least 5 days. Leave it on if it won't turn off and allow the battery to run down, attempting to turn it off may do more damage than leaving it to run down.

Salt water & electronics are bad...really bad. Even if you successfully dry the phone out, I'm afraid you can kiss it good-by. The residue from the salt water will slowly eat the insides away. Best to cut your losses & do an out of warranty replacement. I wouldn't waste any more time messing with it.

Yep, iPhones are not designed for user DIY. See if you can find a third-party repair shop where you are. If you do, plan on replacing the battery...but have them disassemble, then soak in distilled water, then dry out, then put everything back together. Bear in mind, the odds are low this will work, but I can't think of anything else for you to try.