If you can, dismantle the phone (this would be a brave thing to do).
Originally Posted by Xeron
Further drying may or may not work depending on whether there were dissolved salts or a little oil in the water. There is a possible remedy which I can't guarantee but may work. You would need to obtain some pure iso-propanol. Check that it does not affect the case (it shouldn't) then remove back, battery and sim card and soak the phone thoroughly in the isopropanol until you see it is getting between glass and lcd. You will need to spend time on this, shake the phone around and swill it. Wear rubber gloves, obviously. The isopropanol will dissolve small amounts of water and any oil or grease.
Now leave the phone propped up at different angles to allow it to drain thoroughly. Then set a warming oven to no more than 45C and leave the phone in overnight. This will evaporate all the remaining isopropanol.
Isopropanol is used as a cleaner for many electronic assemblies and they are almost all rated safe for use with it.
Almost as good is pure industrial alcohol but it is very hard to come by.
When I did my training as an electronic engineer, many years ago, the technicians used to clean up equipment thoroughly by first washing with DI water containing a few drops of detergent, then with pure DI, then with isopropanol and finally with diethyl ether. Something that looked as if it had been in a war (sometimes it had...) would emerge looking new.