What is the reason?

The first time in Bulgaria on the Golden Sands a few years ago, then regularly on vacation …
And now the last two weekends at the lakes near where I live. Many English people seem to be on vacation in Berlin and the surrounding area. More than usual.

Sorry, I wasn’t brave enough to ask directly ;-)

Why do “you” take your bottle of beer with you into the water while swimming, to the toilet while peeing, …
Different place, different people and groups (always speaking English) doing the same thing.

The wondering Raik

Why do “you” take your bottle of beer with you into the water while swimming, to the toilet while peeing, …

I wouldn’t, because it’s kind of yucky, but I would imagine that it’s a fear that something left unattended might be swiped by somebody else. Or, worse, that somebody might do something unpleasant to it “for a laugh”.

It was quite common at boarding school for somebody (usually a prefect) to take somebody’s drink, drink some (or at least, pretend to), and then hand it back while informing the person “I spat in it”.
Because of that, I wouldn’t let a drink out of my sight. But, damn, I’d finish it before heading to the toilet.
And holding a beer while swimming? WTF? That sounds like a Nigel Farage caricature!

I wouldn’t, because it’s kind of yucky, but I would imagine that it’s a fear that something left unattended might be swiped by somebody else. Or, worse, that somebody might do something unpleasant to it “for a laugh”.

Then this is a “mass syndrome” in people under 30 years old …

Because of that, I wouldn’t let a drink out of my sight. But, damn, I’d finish it before heading to the toilet.

I also ;-)

And holding a beer while swimming? WTF? That sounds like a Nigel Farage caricature!

Men and women…

Then this is a “mass syndrome” in people under 30 years old …

I would agree, but probably applied to people even older.
The fashion for drinking from the bottle seems to have started when “spiking”¹ a strangers drink became common in clubs.

The habit replicated in pubs was often greeted with derision by the landlord of the pub and in one case he bought babies bottle teats to fit on any bottle not poured directly into a glass.

And holding a beer while swimming? WTF

I actually did that once – Malta, July. I wanted the beer and it was too damned hot out of the water even though it was only 10am. The middle of the day was a hide deep indoors.

¹ Despicable practice of adding some drug to the drink.