Hi @gachaz ,
What is the make and model number of the phone?
As you know 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.
First do not try to turn on or charge your phone and then you need to remove the battery as soon as possible from the phone to minimize any further damage.
Then you need to dis-assemble the rest of the phone and clean all the affected parts using Isopropyl Alcohol 90%+ (available from electronics parts stores), to remove all traces of corrosion and water. Do not use "rubbing alcohol" as in some cases this is only 70% IPA or less, can contain scents and is not as effective. If you do check the label to verify the amount of IPA.
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.
Hopefully after you have done all this the phone might possibly work correctly again.
You don’t state the make or model number of the phone so search on ifixit to see if there is a repair guide for the phone which may be of some help in the dis-assembly process.
If cleaning it yourself seems too daunting, take your phone to a reputable, professional 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.