Although drawings are traditionally mounted, they can also be floated or surface mounted. All delicate materials such as paper need to be kept away from the glass. The attractive edge of watercolor paper often demands exposure and floating and surface mounting are great options.
In the case of floating watercolors a box frame solution is required so that there is enough depth to keep the paper from touching the glass.
When surface mounting drawings and watercolors and to expose the edge of the paper you can add a single mount or a double mount surround. This should provide sufficient depth to keep the paper away from the glass.