We used this idea for "Desert Week" but I could see you this also as a great activity as a "beach" project.
The materials are simple- watercolors, water, fine sandpaper, and paintbrushes. We did discover a slightly better quality paintbrush works a little better.
The boys each started adding colors to their sandpaper. We did pretty quickly discover that it was better to move the brush in only one direction at a time and not back and forth.
While the two littler guys primarily explored just adding paint to their pictures, Big Brother worked on trying to create a dessert scene. He used some of the books we'd been reading for inspiration.
I loved how the watercolor looked on the paper, it reminded me of sand paintings you might see in a desert cave - so I couldn't resist trying my hand at a desert painting too.
We'll definitely be exploring this medium some more, looking for other ideas for our paintings.