Is it fastest if I take it to the Genius bar or send by post?
I have spilled some water with yeast in it on my macbook. Turned off and let it dry for two days but it cannot be turned on. I actually do need it, especially various data I have in it as soon as possible. I thought I had backed it but unfortunately not.
I stopped buying newer Macs when the SSD drive was integrated (soldered) to the main board. Still using a 2010 model. If you can remove the SSD on your version then buy another off Ricardo and swap the drive in.
Assuming the worst, soldered on, the water has most likely damaged the power on switch. It has happened to me several times over the years, different Mac but similar keyboard. You can start the Mac by shorting 2 pins on the board, running with an external keyboard, and use DD or cat to mirror the internal drive to an external drive. Then do a normal time machine backup, try and restore it onto the Mac from Ricardo.
You need the exact same model and version, you can check from the serial number on the base cover. Here is an ifixit which covers starting the air with a dead keyboard due to water damage. https://www.ifixit.com/Guide/How+to+…+Switch/101532