One common solid material that can float on water and is both sustainable and relatively cheap is wood. Wood, in the form of logs, planks, or small wooden objects, is less dense than water, which allows it to float. It’s also a renewable and sustainable resource, as long as it comes from responsibly managed forests.

Here are some key points about wood as a floating material:

1. Sustainability: Wood is a renewable resource, and when harvested from sustainably managed forests, it can be an environmentally friendly choice. Many countries have established forestry practices to ensure responsible wood sourcing.
2. Low Cost: Wood is generally affordable and widely available in various forms, making it an economical choice for applications where floating materials are needed.
3. Versatility: Wood can be used for a wide range of applications, including building rafts, boats, docks, and various water-based structures.
4. Eco-Friendly: Unlike some synthetic materials, wood is biodegradable and does not contribute to long-term pollution when it eventually breaks down.

While wood is a good option for floating materials, it’s important to note that specific types of wood and the treatment or coatings applied to the wood can affect its buoyancy and durability in water. Additionally, other sustainable and low-cost materials like cork, certain types of foam, and some recycled plastics can also be considered for floating applications, depending on the specific requirements of the project.