I am going scuba diving in a month and need to take some underwater photos. I am considering buying a hard plastic case for a digital camera which run around $250. Or I am thinking about trying these soft plastic cases that run around $30. Has anyone used either of these before? Did the photos turn out okay? I will be going in shallow water, probably no deeper than 60 feet. Do you think I need an extra flash to take good pictures?

You get what you pay for. It’s your camera. Personally I’d spend whatever it takes to keep that camera safe and able to take decent shots.
As for a flash, you’ll probably find that you’ll want to put a piece of tape over the flash if there’s no option for turning it off. Most of the time, a flash will produce back scatter in images ( looks like a snow storm) as the light from the flash hits suspended particles in the water and bounces back. If your camera and housing allow it, the use of what’s called a strobe will be better. It’s a type of flash that’s mounted and pointed off to the side, reducing back scatter.