It could be fogging on the lens as suggested above or it could have just been the way the sun reflected off of the water.
Another thing that could cause something like this - is if something came between the camera and the subjects that were close enough to the camera to come out as just a sharp drop in contrast. This would be a little like shooting through a fence with a telephoto lens – If the fence is close and the subject is far off the fence could be rendered so far out of focus that it is invisible but it will still take away contrast from the subject.
Could this be a towel or swim suit that just caught the edge of the frame?
Are these pool shots the only shots that you have seen this?
Have you done any shooting with that camera and lens since?
I would just shoot as you normally do and see if this happens again.