Sky, water and the monster did not get along
If taken separately, the three things are done well enough, but the first two and the third fused together make a mess.
Sky and water in fact (and as for water i'm talking about the part where the monster comes out with the torso, because near the tail it's not done just as well) show a different staining pattern than that of the giant water snake.
While the landscape is very vague, the mythical beast is much flatter and has very defined boundaries, and if blurred, as you did for example at the end of the tail, everything would had an airr of mysticism and all together would have been more pleasing to the eye. But as it is now there's a kind of step between the two elements of your drawing
4/5 - 7/10