Hi @bjoerngroehn
Depending on which side of the headphones fell in the water, it may be more a case of seeing what damage due to any water ingress has occurred to the electronics in the headphones first, before seeing if it needs a firmware reflash.
The headphones are only water resistant and not water proof.
''The impurities in the water causes corrosion and provides circuit paths for the electricity which were not in the headphone's operating design and could damage the components. The corrosion starts immediately and is ongoing until it is completely cleaned away''
''***Do not continue to charge or to turn on the headphones*** and then ***remove the battery as soon as possible*** ''to minimize any further damage.
Then you will need to dis-assemble the rest of the device and clean ''all the affected parts'' using ***Isopropyl Alcohol 99%+*** (available at electronics parts stores) to remove all traces of corrosion and water. If possible do not use "rubbing alcohol" as in some cases this is only 70% IPA or less, can contain additives which may leave conductive residues and is not as effective. If you do have to use it check the label to verify the amount of IPA. The higher the percentage of IPA the better
Here is a link that describes what happens. [[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. The ends of any flex cables and their connectors need to be cleaned as well
Hopefully after you have done all this the headphones ''might'' possibly work correctly again.
Here's a battery replacement [https://www.youtube.com/watch?v=dju63rbHLFM|video] that may help as it shows how to open the headphones etc