You don't need a mixture to separate water and seashells. You can simply use gravity.

Here's how:

1. Let the mixture settle: If you have a container with water and seashells, let it sit undisturbed. The seashells, being denser than water, will sink to the bottom.

2. Pour off the water: Carefully pour the water from the container, leaving the seashells behind.

This process is called decantation. It's a simple and effective way to separate a mixture of a solid and a liquid.