As you know water and electronics and electricity are not a good mix.
The minerals in the water (all water except distilled water) cause corrosion and water being a good conductor provides circuit paths for the electricity which were not in the phone's operating design and could damage the components. The corrosion process starts straight away
First do not turn on your phone and then you need to with the phone turned off, remove the battery as soon as possible from the phone to minimize further damage.''
Then you need to dis-assemble the rest of the phone and clean all the affected parts using Isopropyl Alcohol 90%+ to remove all traces of corrosion and water. Do not use "rubbing alcohol" as in some cases this is only 70% and is not as effective. If you do check the label to verify the amount
Here is a link that describes the process.
Electronics Water Damage
You may have to replace the battery as it is very hard to recover a battery from water damage.
As always with electronics, especially surface mounted PCBs 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.
Here is a link to the ifixit guide(s) for repairing your phone.
LG G5 Repair
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.