XGE pg 164, Scatter
You teleport each affected target to an unoccupied space that you can see within 120 feet of you. That space must be on the ground or on a floor.
You're on a boat and cast scatter on someone else on the boat. If the water is less than 120 feet deep and you are looking over the side of the boat, could you teleport them to the bottom?
You're on an active volcano next to a pool of lava. Can you scatter them into the lava? You technically can't see the ground under it, so if you can send someone underwater could you not do it with lava? Or does lava count as ground since you can technically walk on it (assuming we're using real lava not action movie lava)?
You're next to a frozen lake. Can you scatter someone under the ice?