There is a very slight possibility that the phone may come to life, saltwater is not good for electronics. If you remove the battery and soak the phone in fresh water for 5 minutes, then rinse with warm water, until all salt is removed. Let it dry in a warm place, better if you can "blow" the water out with compressed air (computer key board duster cans). I have used a hair dryer on low temp/speed at a distance of 18 inches (do not heat the phone up). Next place the mostly dried phone (still may be wet on some internal components) in a sealed bag of white rice to absorb and left over moister. The battery may work if cleaned and charges, but salt water and batteries definitely do not mix well together.
I have saved two android phones using this method, again the chances are slim and go down exponentially as time in water increases.
You can go online and check Gazelle or Smart phone tradein or sites that buy them working or not. But first google 'lost or stolen cell phone list'.
There are a few sites that you can plug in the iphone serial number to see if it has been reported lost or stolen. If it IS on the list, no one can give you money for it.
If the serial number is not visible on the outside of the phone, it can be found on the card slide by putting a paperclip in the hole and getting it to pop out.
Chances are it would be on the list if the person had cell phone insurance. I found out the hard way when I sent a couple of 'found' phones in a couple of years ago.
One was on the list. I sent one in that looked like it was run over by a tractor and got $40 for it.
Good luck