You can create the surface by using the displacement modifier with a cloud texture at a low setting.
Or by going into edit mode, subdivide to a number and using the fractal option to displace vertices. Then smoothing it with a subdivision surface modifier.
Both methods produce nearly the exact same result. Pic related, Left is fractal method, right is texture displacement.
In terms of setting up materials for a game engine, I'm not totally sure. I don't use blender's game engine so I don't know what types of materials it's capable of. You should be able to get a decent glossy material out of it though.
Most of the reflections come from the angle the viewer is at, and how low the moon is to the water. Play around with placement to find what works.