charlise,
It's a flaw with the way modern humans access risk. We have cushioned our daily lives with the trapping of modern society that generally make most of what we do everyday quite safe and consequence free. As a result, when we access the risk of any given particular activity we often incorrectly assume that nothing bad can or will happen. If it does go souith, then we unrealistically expect "someone" will come to our rescue.
Hikers in the back country make these false assumptions all the time, avalanches, getting lost, injured do happen, much to the surprise and dismay of those involved who are not prepared to deal with the possibility. Swimming in the ocean is no different. We do it all the time without so much as a second thought. Usually, everything is fine, until it's not.
Generally speaking, humans are no longer capable of making good, practical and honest decisions a lot of the time. In many cases we are making potentially life altering decisions without a foundation of underlying knowledge upon which to bases these decisions. It basically comes down to Darwin at work...