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 starts immediately and doesn’t stop until it has been properly cleaned away.
Do not try to charge or to turn on the phone and then remove the battery as soon as possible so as to minimize any further problems.
Next you need to disassemble the rest of the phone and clean all the affected parts using Isopropyl Alcohol 99%+ (available at electronics parts stores) to remove all traces of corrosion. Do not use "rubbing alcohol" as in some cases this is only 70% IPA or less, 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 concentration (%) 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. Also remove any shields that may be covering some components and clean there as the water may have got in under them as well and corrosion is occurring there. The ends of any cables and the connectors need to be cleaned as well.
Hopefully after you have done all this the phone might possibly work correctly again. You may have to replace the screen but it pays to try the above first
Here is a link to a teardown video of the phone, which may help
If this process seems too daunting, take your phone to a reputable, professional mobile phone repair service, experienced in liquid damage repair and ask for a quote for a repair. If you decide to do this, do it sooner than later.