For that I now have the perfect device.
The waterproof Bluetooth speaker comes in a pretty cardboard packaging and has thus passed the first impression solid.
The loudspeaker has been well finished and this is still an understandable manual plus charging cable with.
The coupling to the mobile phone via Bluetooth was not a problem and went very quickly.
However, the sound is much better than that of a smartphone a little tinny, but that is okay because it is no longer perceived by the rushing water.
The operation of the buttons work properly. The speakerphone I have not previously been tested. I'm sure it works just as well as the rest also.
Can attach to the speaker by using a suction cup.
The integrated battery is completed only once gone with me to run out and the estimated after 4-5 hours of run time. Charging takes about as long as the smartphone and running easily via the included USB cable.
Conclusion: Very good value for money I would buy again!