Strange UFO like bird over sea in Mexico

On January 22, 2015, a holidaymaker was near the sea in Mexico. Suddenly, he noticed strange thing that was flying just over water. He took some photos. What is it?

Witness report:

I was relaxing on the patio of the ocean home I rented on vacation in Isla Mujeres, Mexico. I had noticed the large Frigate birds and wanted to get a picture of one which was near. I took 3 pictures with my IPad and when I looked at them I noticed this strange object descending into the water on the 2nd and 3rd pictures. I had not seen the object prior to that review. You can see it at about 50 feet above the water on the 2nd picture and then it appears to be entering the water a few seconds later in the 3rd picture. My friends and I could not explain the photos and none of us actually saw the strange object. I really feel that I unknowingly photographed a UFO.