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What is it
According to Polyurethane.org, the online center for the polyurethane industry, polyurethane is formed when an alcohol with more than two reactive hydroxyl groups per molecule reacts with a diisocyanate or a polymeric isocyanate. This creates a range of materials that are durable and cannot be melted or reshaped like thermoplastics. Among the materials created are foams, chemical-resistant coatings and sealants.
Foam
Polyurethane foam is used in a number of objects that come in to contact with water, but as a core rather than as a material exposed to the elements. Surfboards can be shaped with a polyurethane core that is both lightweight and floats on water, then covered in fiberglass to prevent water filling the gaps in the foam. Similarly ships use polyurethane foam between the inner and outer hulls to aid flotation and limit sound passing back and forth.
Coating
Inflatable life rafts can be coated with a polyurethane spray to ensure that water cannot leak in and air cannot leak out. This coating also protects the craft from rough abrasions and provides limited protection from objects in the ocean that can damage the raft. Suspension bridges are often coated in polyurethane because it creates a barrier between the support beams and the weather. This prevents rusting due to rain or water spray and also makes the beams easier to clean.
Sealant
As a sealant, polyurethane is used in the creation of products as diverse as the finish on a pair of shoes and the construction of a car. It prevents water leaking into shoes and bonds the windshield of a car to the frame without letting any water in. As an adhesive, it is used in a mixture with wood chips and saw dust to form fiberboard because once dry it will be waterproof. It is also used in plumbing, where waste water is a constant concern, construction and road repair. As an added bonus, it can also be painted on to ensure that the sealed area matches the area around it.