Strong winds sweep the coast of the Pacific northwest in summer; Bahía Salinas [1] is recommended and has two windsurfing centers. Inland, Lake Arenal [2] is paradise, with 23–35 kph easterly winds funneling through a mountain corridor year-round. Strong winds rarely cease during the dry season (Dec.–Apr.). The lake is one of the best all-year freshwater windsurfing spots in the world, with two dedicated windsurfing centers.
Links:
[1] http://www.moon.com/destinations/costa-rica/guanacaste-and-the-northwest/the-far-north/bahia-salinas
[2] http://www.moon.com/destinations/costa-rica/the-northern-zone/lake-arenal