Update, Bones! I created a muddy bones material with Quixel Mixer using Megascans and sculpted a skull in zBrush to add some extra depth. The material also has a very basic panner animation on it, to give the illusion of moving water.
Update, I replaced the Megascan material by creating my own Substance Designer material for the ground and added some vertex blended water & skulls. I made the shadows softer and added more contrast, like @"Cactus on Fire" suggested :) I also created some small assets, like the bones and ribs and polished the scene…