Hello,
Considering that the phone has sustained some water damage, most likley the camera module that is attached to the motherboard is damaged. Luckily, there is a tear down guide regarding this device:
Asus Zenfone 2 Teardown
The guide above will allow you to remove the individual camera modules so you can further inspect them to see if they are damaged. If there are discoloration on the connector pins, or corrosion of any type, the camera module may be damaged.
If they are damaged, you can replace the front and rear camera modules with the two parts available on eBay:
https://www.ebay.com/itm/Asus-Zenfone-2-...
and
https://www.ebay.com/itm/OEM-UNLOCKED-AS...
I would also advise on looking at the motherboard to see any visible signs of damage that may have been from the initial water damage. If the board has any other corrosion, the modules might not be the cuprit.
That being said, there is also the possibility it is operating system related. If you are aprehensive about opening you our device and haven’t already tried to factory reset the phone, that option would be the most non-invasive method, and would allow you to eliminate one possibility.
There are several revisions of the Zenfone 2, so if you can provide an exact model number, we can verify if the parts above will be compatible.
Thank you,
Complete Technical Solutions Inc