My sighting at Lake Superior

Now that I think back on this, I find it highly likely that what I saw was a weird wave, but who knows? Here's my story.
A few years ago (I don't remember exactly how long ago it was, but I was in middle school when it happened, so it was at least 6 years ago) my family went to Duluth, Minnesota for vacation. One of the days we were there, we took a drive down the north side of Lake Superior. I was looking out the window, when through a break in the trees, I saw a weird black thing that looked like this:

It appeared to be about 10-20 feet long. I only saw it for about a second before some more trees got in the way, so I don't know if it moved. However, when we drove back, there was no black shape there, so it wasn't a rock.
Like I said above, I think what I saw was probably a wave, but it resembled descriptions of the Loch Ness Monster (which are described often as 3 humps). Also, there's a resemblance to the Champ pic: