Reading about Philip Greenspun's bout with Decompression Illness (The Bends) in Australia has convinced me I made the right decision when I stopped scuba diving long ago. I took a scuba course in grad school, and became certified, but our ocean dives were freezing cold (diving in the Pacific in December is no fun) and I had ear and sinus problems for weeks after our dives. Sitting 30-40 feet below the water's surface also taught me that no matter how much training or preparation I had, a series of minor mistakes could lead to death - that being in trouble 40 feet below the water was as hopeless as being in trouble on the surface of the moon.
The vulnerability was too much to bear, especially when weighed against the marginal fun or pleasure scuba diving offered. I'm a snorkeler for life now.