The unnamed woman, who was on holiday in a popular area of southwest Turkey, was flung head first from an inflatable into a nearby motorboat and died instantly, Die Welt daily said on Wednesday.
She was with a 14-year-old German girl, who sustained severe injuries and has been taken to hospital.
The accident is, the paper said, thought to have been the boat driver's fault as he turned very sharply, purposefully tipping the two girls off the inflatable being pulled behind the boat - a popular motorboat manoeuvre.
But, 100 metres off the coastline, he turned too close to another vehicle, catapulting the pair into it.
It has not been revealed where in Germany the pair were from, but it is known that they had been holidaying in Sarigerme, a popular tourist destination in the Mugla province.