A friend of mine has built the largest homemade submarine in the world (http://en.wikipedia.org/wiki/UC3_Nautilus) and they're clearly dangerous vessels, exactly because of what you're saying: If something goes wrong you're under 100 feet of water with no way of getting out. Even though UC3 Nautilus has a lot of built-in safety there have been a few situations that I would consider much more dangerous than sailing a dinghy across the bay of Biscay in a force gale 12 storm.