Tim, this is an excellent observation!

Every pilot is trained on how to deal with a number of common optical
illusions. Flying over water is one of them - it's often quite
difficult to tell how high you are over it, and you are often "tricked"
into thinking that you're lower.

Floatplane pilots account for this effect by keeping the aircraft in a
landing attitude (nose-up) from their approach until they reach the
surface (search for "glassy water" and you'll see). Land-based
airplane pilots use the same technique when performing an emergency
landing in the water.

Your plane wasn't landing in the water, but you caught one of the
optical illusions!