It's not all that rare to see sharks when diving off the east coast of FL.
However there are only two sharks I really fear vs. respect. Those are the Bull shark (I have seen them UW), and the great White (never seen one of those monsters). Great White sightings off the S. coast of CA. while diving are very rare.
As you can tell from the responses on this thread, most of us have never seen a Great White, and likely never will. We are not on their diet, so most sharks leave divers alone. The exception to that rule is when spearfishing, or chumming to attract sharks.
The best thing to do if you do see a shark is to keep an eye on it, and exit if it displays hunting or defensive postures. If you want to know what those behaviors look like, I'd use the internet search option!
I would not limit your time in the water because of a fear of sharks. They generally leave divers alone. I've seen a LOT of nurse sharks in FL, and they are likely more worried about us. The one exception I experienced was while spear fishing. I think my spear must have had blood on it (not that I could tell), but I had a good sized nurse shark following me around on a dive during the day. Not typical Nurse shark daytime behavior. Not the company I prefer, but it behaved it's self, and kept some distance even if it was shadowing me.