I have taught underwater photography as well as land for many years in classroom. Any workshops I found always taught me at least something new or had me thinking about a different way to do things. Learn both from the instructor as well as others in the workshop - I find my students give me the best inspiration and ideas! The more you learn, the better you'll be although practice is key.
Also be sure you have great buoyancy. I rewrote the SSI photo and video manual last year and tried to incorporate as much as possible to help make your underwater photos look better and gave hints and advice like having good buoyancy (so best if good diver first before taking a camera underwater), how to approach subjects (and find them! Luck does play a part with underwater photography! ; )).
I also highly recommend taking a photo class (like through an adult education) as things like lighting and composition, which are the two most important factors in a photo no matter where it's taken, are important to understand if you want your photos to look great. I did put in the SSI text (although they did edit it a lot) about lighting and composition underwater (and how to place strobes to avoid backscatter and get good lighting!).
Last but not least, a little bit of photo editing is important too. You can't make a bad photo look great but can make a good one look fantastic with a little editing!
One of the things I love about doing photography is you can always learn more and being on workshops is a fantastic way to do so!
Good luck with your diving adventures (and getting great subjects to "pose" for you!).