Since this happens again and again and there is a lot of useful info on the forum, but people usually read it too late I’ll gather it here in hope that you read it before this happens to you.
This is a wiki so feel free to add info by editing the post.
First things first: Turn off and take apart
If your phone comes in contact with water and there is even the slightest chance that some droplets or moisture entered it then turn off the phone immediately. The fastest, and only in this case the best way, is to take out the battery.
If the phone is off then not much damage can happen to the phone, but you should still try to take it apart fast so the water doesn’t spread too much. So get to a place where it’s safe to disassemble the phone. If you’re in transit atm maybe just take out the battery, take off the cover and screen (FP2) and store the parts somewhere dry (If you’re out in a rainstorm and all your clothes are soaking then it’s better to leave the phone assembled for now - except for the battery, just to make sure the phone doesn’t turn on again).
Once your in a safe and dry place disassemble the phone as far as you can - so in the case of the FP2/FP3/FP4/FP5 that means at take out each individual module (Taking apart individual modules is not advised unless you are experienced in it). In case of the FP1 disassemble as far as you believe you can reassemble again.
Check out iFixit for help taking FPs apart.
Let it dry for days
As important as it is to turn the phone off quickly it’s just as important to not “test” it until it’s super dry. A good rule of thumb is: Let it dry for at least two days even if you didn’t see any signs of moist inside the phone and if you did see some, let it dry at least twice as long as until the signs are gone.
Don’t use rice
or any other substance that can leave residue inside the phone - this will only cause more damage.
What to use instead
If you’re not in a hurry to use your phone (don’t be!) it’s safe to just let the phone dry in the air.
One thing that was recommended repeatedly on the forum and seems to be safe is Silica. This is the substance that often comes as small spheres in a little bag packaged with anything you buy somewhere that shouldn’t get wet during transit.
Another option is to put all the parts (in batches) in a hearing-aid dryer, as those are designed for removing moisture from electronics safely. Otherwise it’s not recommended to dry the phone with heat. Too much heat can cause damage.
Yet another option is described in this iFixit article. The basic idea is to displace water - with all the (salty) minerals and small solids floating around in it - with another liquid that dries without leaving such residue, namely 90%+ Isopropyl alcohol. If you let the water dry in-place, what can happen is that the water will be gone, but the previously solved minerals therein won’t be. They can then cause contact issues or corrosion later on.
Check for damage (still before turning on)
Check each part of the phone for signs of damage. Especially check the connectors to see if there is some sediment on it. This can come from the minerals in the water. Carefully clean it off with distilled water or alcohol.
If you are reading this guide too late and used rice or something similar then get an air-can and try to blow out all rice-particles too.
Check the water-sensitive indicators on the phone. These are white squares or dots which turn red when in touch with water. This may help you check which modules got affected with water (which doesn’t necessarily mean the module doesn’t work anymore, nor doesn’t it mean the water didn’t come up to the module, although it should be a useful help).
There should be one on each module (for the FP2 at least) or on top and on the bottom of the phone (FP3). (no info about FP4 or FP5 yet)
Click to see example photo on FP3
photo taken from How “waterproof” is the FP3? IP Rating - #34 by anon9989719
In red: a red water indicator due to water.
In blue: typical traces of water damage on the protective shields
Click to see water indicators on FP2 parts
In green: normal indicators
In red on the bottom module: red water indicator
(on the screen it’s under the protective shield that has to be taken off carefully)
Only test the phone if you’re absolutely sure
that it’s as dry as can be and you cleaned it as well as possible.
Now test all functions of the phone to see what is damaged.
For the FP1 here is a guide to figure out which parts might be damaged if a function doesn’t work → fp1repairguide
If something doesn’t work, but you don’t know which part is responsible try to find someone near you with the same FP model (1 and 1U counts as basically the same) who will let you test some of their parts in your phone and vice versa to find the culprit.
Check out the fairphoneangels for help with testing.
If you turned on the Phone too early
and something doesn’t work as it should: Turn it off right away and let it dry for longer. There is a chance that it will work again then.
To paraphrase a mnemonic about hypothermia:
A phone is only dead if it’s dry and dead.
The FP3 and older phones are not waterproof, the FP4 and FP5 are partly waterproof
If you have a FP1, FP2 or FP3 and have often contact with water think about replacing it with a FP4 or FP5. The FP4/FP5 are IP54/IP55 certified. See IP code - Wikipedia for what does it mean.