Bottle in flight hand luggage: that's how it works
If you do not want to give up your own drink in hand luggage when traveling by air, there is a trick:
Take the EMPTY bottle and fill it up at the tap AFTER the security check on the toilet!
In Germany, as in most European countries, tap water is the best-controlled food and can be eaten from tap 1a. So you are at least prepared for the outward journey.
If you do not like tap water, you can use effervescent sachets, effervescent tablets or similar. pack up - they can be taken along - and spice up his flight beverage with it. So you do not depend on the expensive shops in duty free.