I had the same problem with an image I did of the 1936 Sedan from DAZ, when I used raytraced reflections - the car is supposed to be shiny (duh!), hence hte raytracing; set your view for "Wireframe" and see if those rectangles don't match the polygons of your water. It happens when smoothing is on on a surface that has relatively large polys, from what I can see; you might need to use a reflection map, instead.