Thermoset polyurethanes remain stable even when immersed in water for very long periods.  They absorb practically no water and show no negligible swell in volume even after prolonged immersion.  In fact, these polyurethanes are used successfully in water-lubricated bearings, where a slight increase in volume would cause seizing.

To see the different types of Thermoset Polyurethanes, please see Choosing the Best Polyurethane Material.

For a comparison of Thermoset Polyurethane – Vs – Thermoplastic Polyurethane, please see Thermoset Urethane -Vs- Thermoplastic Urethane.