The following options can help prevent water from eroding the soil at the beach:
- Shells: Shells can help stabilize the sand and reduce erosion.
- Planks: Planks can be used as barriers to slow down water movement and protect the soil.
- Sea walls: Sea walls are constructed specifically to protect coastal areas from erosion caused by waves.
Therefore, the correct answers are: shells, planks, and sea walls.
Sand, while necessary for beach composition, does not prevent erosion on its own.