I've been diving since 1969 & have had very few incidents (other than losing my friend & diving buddy to a massive heart attack while swimming on our way back to shore during a beach dive) I live in Massachusetts so most of my diving has been in local waters during the summer/fall months. I must say that what scares me the most is not sharks, Killer Whales or what others (usually non-divers) think would be dangerous down there........It is getting tangled in seemingly inocent & ubiquitous fishing lines, lobster trap lines, dive float lines, etc!! Yes, I do carry a dive knife (which I would never dive without but which, luckily, I have never had to unsheath) for just that eventuality, but I still try to steer clear of any such lines anyway. (Funny how something so innocent on the surface can be so dangerous down below!) Just wondering what things cause concern to other experienced divers ??