Swimming and boating on Mars
Was re-reading C. S. Lewis' "Out of the Silent Planet," and noticed something I hadn't before. His character makes a point of showing how boats on Mars have very little draft, and he has trouble swimming because he doesn't sink very far into the water.
Now, Lewis wasn't a physicist, but it occurred to me that the amount of water a floating boat or person would displace should be exactly the same on any planet in any gravity (assuming the makeup of the water is the same, of course).
I could see how the casual thinker might easily think otherwise, though.