Well, they always say that a poor workman blames his tools (!), so the only thing I can say is that I blew it and the camera covered for me. When I got the camera for my birthday last month, I was reading Ken Rockwell's site (it came up when I googled a question). He seems knowledgeable so I set up the camera as he suggested as a starting point. Auto ISO was one of the suggestions so it's where I've been shooting so far. But S mode was a mistake; I usually shoot in P mode but I was out with my wife shooting away, spinning dials, and that's what resulted. I didn't even realize that had happened!
I am going to get out of Auto ISO and move more toward manual operation. I probably should have been at ISO 64 or 100, huh? I've always liked shooting shutter priority for water shots but I know that in this case I just mistakenly had the camera at S instead of P mode. It's amazing how much the D5100 compensates to get the exposure...and I'm just learning how to work with digital. My previous camera was a Canon A-1 that I had three nice lenses for, and this is my first DSLR. I had a Coolpix for my very first digital camera. I am just loving learning about the DSLR world! Reading Bruce Barnbaum's "The Art of Photography" now and my Christmas list has Ansel Adam's 3 books on it, I think they're called "The Camera", "The Negative", and "The Print".