Can you post a screen shot of what you're brush looks like, and one of your attempt to apply the brush? Leave the tool selected, so we can see the settings.
How did you create your watermark brush? If you created the brush from an image that wasn’t solid black, it will paint at an Opacity that matches how it was captured.
IMO if you want to add a watrmak manually a brush can work well. However if you want to batch watermark images you would be better off using a png and a Photoshop script to place size and posttion in your watermark onto your images.