A watersail is a

sail A sail is a tensile structure, which is made from fabric or other membrane materials, that uses wind power to propel sailing craft, including sailing ships, sailboats, windsurfers, ice boats, and even sail-powered land vehicles. Sails may b ...

hung below the boom. It is used mostly on gaff rig boats for extra downwind performance when racing. Often a watersail will be improvised from an unused foresail. Its psychological effects may be more effective than its aerodynamic ones.

References

Sailing rigs and rigging {{Sailing-stub