Or, maybe the remains of the bird predate the presence of the body of water. Maybe it ate marine reptiles. Maybe it merely died in the ocean due to being attacked by a predator while floating on the surface. There is not enough information to support anything beyond what is seen in the remains. It was a carnivore. Speculating about dietary habits is just guessing, with no more factual basis than saying it was green with brown spots and nested in palm trees for camouflage amongst the coconuts.
“Speculating about dietary habits is just guessing....”
Not necessarily. Trace amounts of chemicals and biochemical remaining in bone not fully mineralized or in mineralized fossils of bone can reveal the dietary composition of the animal’s prey.