The ornamental design for a soda-water preparing device, as shown and described.
1. Soda-water preparing device
1.1 : Perspective (front, top, right)
1.2 : Front
1.3 : Back
1.4 : Right
1.5 : Left
1.6 : Top
1.7 : Bottom
The shading used in the drawings represents contours and does not illustrate ornamentation or decoration on the surface of the article. The broken lines depict portions of the soda-water preparing device that form no part of the claimed design.