One solid material that can float on water, is sustainable, and relatively cheap is expanded polystyrene foam, commonly known as Styrofoam.
Expanded polystyrene foam is a lightweight and buoyant material made up of small foam beads. It is produced by expanding polystyrene beads with the help of heat and a blowing agent. The resulting foam structure is composed of interconnected air-filled pockets, which give the material its buoyancy.
Styrofoam is widely used for packaging, insulation, and disposable food containers due to its low cost and insulating properties. It is also commonly used in flotation devices and as a buoyant material for various applications.
However, it's worth noting that Styrofoam is not environmentally friendly. It is not biodegradable and can persist in the environment for hundreds of years, causing pollution and harm to ecosystems. Alternative materials that are both sustainable and biodegradable, such as certain types of natural cork or specific biopolymers, may be preferable from an environmental standpoint, even if they are slightly more expensive.