the pebbles on the ground looks quite real. But the larger boulders in the background is too textured and not enough bumpiness and also too uniformly flat. That's what seems to make it fake. also the water looks great, but too uniform and perfect. Real water has lots of little flaws and it's usually not as clear too (things like muddy water is hard to render i guess).

What immediately stood out to me was the lack of waves. It's clearly supposed to be an ocean or large sea, but there are only small ripples in the water.