Suggestion: hold a linear to the water edge on the top and watch carefully: the water line is concave instead of straight (it may have to be convex, depending on the angle and distance, but never concave).
If you are using a decent stitcher, you can specify top points of the water in all frames as being on a straight line, and the leftmost and rightmost one as being on a horizontal line.
One more thing: the sky shows uneven exposure.
Otherwise it is nice, though too small.