An UFO crashed into my shot!

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The image is from a recent engagement portrait session with a lovely couple Ashley and Jeremy. The couple had just ordered their photos, so I was pulling out the images to print. Suddenly, I noticed there was a black spot on the photo, and I thought it was a bird. After I zoomed in to retouch it out, I was surprised to see an unidentified flying object. There were just a few people on the beach, and I was certain no one was playing frisbee. It would have been a huge frisbee based on the size and distance. No one was throwing anything into the ocean, as I would have noticed it in the background. It was neither a blimp nor a balloon. I took several shots within seconds of the same scene and it only appeared on one of the images. Click on the image to enlarge. What could it be?