With so many low cost cameras out there, why not look at increasing the price of your DUP speciality and include it with your next course?
Many instructors negate the importance of offering this easy to teach speciality, not only is it fun to teach, your students will also find it really rewarding. With the advent of social media they will more than likely post their best images on Facebook or some other form of Social Media, in some cases they will post all 300+ images they take. This could really be a marketing edge for you as all of their friends will get to see why they are so enthused over scuba diving.
You could run a monthly/yearly competition on your company Facebook page with the image receiving the most likes winning a prize. I would recommend that if you do this, to create a different theme every month with a standing theme of the lifestyle of scuba diving. In that way even surface images will be entered and promote you or your dive store in several unique ways, remember to put a rule in that all divers are tagged as this will drive the desired traffic to your page. If you are concerned that one person may get all of their friends to like an image, do not fear as the more activity your page receives the better for you in the long run.