Water damaged iPhone 6 - keeps "search" ing...

Usually when the IMEI number is missing (and consequently cannot connect to the cellular network), then something like your baseband circuit or IC is damaged/shorted.

Your iPhone will need professional repair, with the proper cleaning and micro soldering tools and skills.

You will need to have your logic board removed from the phone, then the shields removed from the board.

Then starts the cleaning process: displace the water, clean the board using a professional ultrasonic machine, rinse, dry.

Then the repair process: replace/repair all apparently damaged components.

Then testing. In which case if something is still not working, the repair technician can look for the specific cause (and may not find it, or at least not in a timely and economical manner).

For this kind of professional approach, be prepared to pay 20% to 30% of the cost of a new device.

Or go to an Apple Store, and pay to have it replaced. I usually prefer repairing. But in case of massive liquid damage, it is usually better to replace the phone.