I like that picture too! I would say the composition is great and you captured the right moment, much needed break from the rule of third composition.
I just added my first strobe to my system but I shot with a loan strobe before and that's what sold me the idea of having a strobe. The strobe, besides adding color, it also add mood to the picture. Do consider attending some UW photo work shop where you might get more explore to different equipment etc. I will be attending the Digital Fiesta (http://www.underwate...italfiesta.com/) and they have strobe and other equipment for divers to try out for free.
Back to your question, post processing should be kept to minimal such as cropping, adjusting expose. That should be it. The goal is to get the picture right the first time so you don't end up with too much post processing, and that's why competition set rules of very minimal post processing. Treat RAW format as a nice to have work flow and don't heavily rely on taking a RAW picture and restore it back because some information just can't be restore back from even RAW format.