From what I'm lead to believe there is no 'normal' watermarking in 1.1.5. You either have watermarking or you don't.
If you have watermark enabled, then by default full-image.php will watermark the images. But that means your normal images will be watermarked as well.
The 'protect full image' option is so that you don't get a direct link to the image. It has to go through some php script in order to get to the image. It does not mean you get your full image watermarked.
With 'protect full image' option disabled, you get a link like www.example.com/zenphoto/albums/testalbum/original_full_image.jpg
If you disable this, all you get is a straight link to the un-retouched original image.
With 'protect full image' option enabled, you will get a link like www.example.com/zenphoto/index.php?album=testalbum&image=original_full_image.jpg&p=*full-image
Of course if you do this you also get the benefits of getting your full image watermarked (but only if you enabled that watermark feature) and recompressed (only if you have watermarking otherwise it just does a file output from the original image).