You can actually display watermarks.
model = Sketchup.active_model opts = model.rendering_options opts['DisplayWatermarks'] = true/false
Yep, there is no function to get/set watermark image, though there is actually a way around it.
"Watermarks are part of a Style. You would have to have a preset Style with the watermark, and change the Style with the API." - quote by Dan.
Create a style or two, add watermarks to each, and set them via Ruby API:
styles = Sketchup.active_model.styles styles.selected_style = styles['Style Name'/style index#]
Reference: Watermark via Ruby API