Water and electronics and electricity are not a good mix.The impurities in the water causes corrosion and provides circuit paths for the electricity which were not in the phone’s operating design and could damage the components. The corrosion is ongoing until it has been properly cleaned away
Rice does not solve the problems caused by the corrosion Rice is for dinner, not repair
First do not try to charge or to turn on your phone and then remove the battery as soon as possible to reduce any further damage
Then you need to dis-assemble the rest of the phone and clean all the affected parts using Isopropyl Alcohol 99%+ (available from electronics parts stores) to remove all traces of corrosion and water. Do not use "rubbing alcohol" as in some cases this is <70% IPA, can contain additives and is not as effective. If you do have to use it, check the label to verify the amount of IPA. The higher the percentage of IPA the better.
Here is a link that describes the process. Electronics Water Damage
As always with electronics, especially surface mounted pcb be gentle when handling and especially when brushing away the corrosion. You do not want to remove any components from the board. You will also have to remove any shields that may be covering the CPUs etc as water may have got in under them as well causing corrosion to occur there.
Hopefully after you have done all this the phone might possibly work correctly again.
Here is the ifixit Samsung Galaxy S6 Repair guide, which may be of some help.
If this process seems too daunting, take your phone to a reputable, professional mobile phone repair service, experienced in water damage repair and ask for a quote for a repair.
If you decide to do this, do it sooner than later.