Paige is very correct about power speeding up the corrosion process so do not try turnning it on. I am sorry to say but do not do put the phone in bag of rice. It is like doing nothing the moisture will still be inside . You need to strip the phone and take the motherboard out. If it has metal plate's ie heat sheilds you will need to very carefully remove them so that you can gain full access to all the componants.
Once the heat sheilds are off, clean the board with old tooth brush and isopropyl alcohol and then do it again. You may also need to put it in a ultra sonic cleaner if you have one or can borrow one. Once this is done try the phone again as I have had quite a few phones that i have had fire back up just by doing the above. The isopropyl alcohol will not only clean the board but it will push excess water out and away from the board.
In extream cases you may need to gently reflow the corroded componants as the contaminant /particles in the water will corrode /crack the soldered joint.
This is why the phone stops working.
Also this is not for the inexperienced though.