It seems to me the board suffered from liquid damage. What i would recommend is inspect the Sim Card area inside the phone for any signs of that substance you saw and if there is get a Qtip with 91% or 99% alcohol and scrub it off hopefully that should do the trick. If it still doesn't work then a component maybe damaged i would personally have to look at the phone. Best of luck!! :)
Do not put it on rice.. even if it sucks the water out you will still have corrosion because of the minerals the water contains. I dont know why people always say rice, doesnt make a difference. Anyways try a full water damage treatment with an ultrasonic cleaner and that should do the trick. Hope this helped
You have to do a full water damage treatment. When its sea water it was wrong of you to leave it in rice for the night because you gave it time to corroed. The reason the phone was getting hot is because the water caused a shortage on the board why you may ask ? because water is a conductive material. You have to understand that sea water contains a lot of substances that may cause corrosion rapidly even tho you just dropped your phone for 30 sec. Anyways after you do a full water damage treatment and after let it dry for around 3-5 hours. Then i would suggest you purchase a new battery and hopefully it will work again. Other than that your phone may just be dead already. Hope this help :)