Clean water is the best case
Tap water and rain carry relatively few dissolved minerals. A board that is opened and cleaned quickly after clean water often comes back with nothing else needed.
The damage from clean water is mostly what happens next: current flowing through a wet board while it is still switched on, and corrosion setting in over the following hours.
Salt water is the worst
Sea water is conductive and aggressively corrosive, and it keeps working after it dries because the salt stays behind. A phone dropped in the sea and left for a day is a much harder job than one dropped in a sink and brought straight in.
If it goes in salt water, rinse it in fresh water before it dries. That sounds wrong and it is genuinely the right advice: diluting the salt is more useful than leaving it concentrated on the board.
Sugary drinks glue everything together
Coffee with sugar, soft drinks, beer and juice leave a sticky residue that bridges components and traps moisture underneath. Connectors seize, buttons stop moving, and the residue keeps drawing water out of the air.
These need a full strip and an ultrasonic clean rather than a wipe down, which is why we quote them after opening rather than before.
What to do in the first ten minutes
Turn it off and leave it off. Do not charge it, do not plug it into anything, and do not keep pressing buttons to see whether it still works. Every one of those pushes current through a wet board.
Dry the outside, take the SIM tray out if it has one, and bring it in. Do not put it in rice: rice draws moisture from the air, not from inside a sealed device, and all it does is buy the corrosion more time while leaving starch in the ports.
It works fine now. Does it still need looking at?
Yes, and this is the part people get wrong most often. Corrosion continues under the shields where you cannot see it. Devices that work perfectly for two or three weeks and then die are the single most common water damage story we hear.
A clean now costs a fraction of data recovery from a dead board later.