I'd imagine that it is at least a few factors. For one, there is a dramatically longer history of sailing and boating such that the occasional accident, even one on a massive scale, doesn't shake confidence so readily.

With respect to airplanes, I think it is again a proven volume/historical problem. Zeppelins (and blimps) have very low maximum speeds and limited capacity. As for airplane accidents, I don't think they are very frequent so much as it is just that they are generally so heavily broadcast (for good reason).