Ironically, when I'm up in a plane, however small, I have absolutely no problem.
Yeah, me too, that's weird isn't it? I DO get nervous in a small land plane as it approaches the runway though. It didn't help that the only one I've ever flown in was piloted by a neighbor of mine that was practicing his landings via "touch and goes", hopping the plane a bit as he did so. I helped him wash and wax the plane once and he took me around the Puget Sound at dusk. Even let me fly it a bit, very cool. It was a little Piper Cub that he was jointly leasing with a bunch of other coworkers at Boeing.
I have flown in a small sea plane too, and I don't have the same problem landing on water. Something about the reassuring look of the softer surface I guess. I'm sure there are just as many potential dangers landing on water though.