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While a flash is best underwater for good color and clear pictures if your target subject is within reach of the puny internal flashes, you can try raising the ISO speed on the camera (if adjustable).
The photos should look fine, and this will give the camera a chance to raise the shutter speed so your photos aren't blurred anymore.
I have a Canon SD870 with the underwater canon housing and an 8g card and I am very happy with it. I forgot the flash diffuser on my last trip and still got decent pictures. The housing also allows you to change quickly between modes and I was able to take videos without losing time to setup.
The camera may not take pictures as good as those with DSLR professional housings (I could not take a decent macro picture to save my life), but the flexibility of changing modes quickly is a way to make up for this and make the most of the camera.
In situations where focus is critical and light is low and you need a slow shutter speed, I have had success with using the self timer. Set it for like 10 seconds or whatever and then when the shutter goes off you are not moving the camera by pressing the shutter button. If you can stablize the camera further with something like a "Ron's Rod" (;) @ PB!), a rock, or even a mini tri-pod and then use the timer, you can get even better results.