Just so everyone knows, you don't have to get the strobe right away, but you should if you can afford to. It makes a HUGE difference in your photo quality, especially if the other option is a built in flash. Backscatter- caused by the flash reflecting off small particles in the water will ruin your photos, a strobe is on an angle which helps to reduce or eliminate this problem.
Just as an example, I have a housing that uses disposable 27 exposure cameras. When using it without the strobe the photos really are just a waste of time, however when I use that same setup with a strobe the photos really come out really nice.
Just my $.02