Chalk pastels alone are a really fun supply but when you mix pastels with water you are able to use them in a totally different way! While you cannot turn pastels into paint, they are actually water soluble which opens up a world of creative possibilities.
Whenever I am using lots of water or wet media, I typically use watercolor paper or bristol paper for my surface. Wetting down those pastels requires a lot of water so you will need a surface that can withstand a lot of water.
Pastels will never completely dissolve or turn into paint but they are really fun to use because you can hold onto those marks, scribbles and strokes.
The colors may change or look different once you add water so I recommend experimenting with those pastels before diving into your project.
You can use those pastels and water in a couple of different ways- Draw first and then add water to the drawing which will dissolve and turn those lines into something that resembles watercolors. You can also wet your surface and then draw into the wet surface. The pastels will be rich and saturated and almost feel like they are melting into the wet paper.
Don't be afraid to layer a variety of materials, supplies and techniques with those pastels. I like layering watercolor paint, ink, permanent maker and even acrylic paint over the top of my pastels.
Depending on your surface and your personal preference, you may (or may not) want to "fix" your work. Keep in mind that spraying a fixative on your pastel work may change the nature of the pigment. Personally, I don't like to fix my pastel work but this is just my preference.