The Huntington resident has succeeded Patricia Aitken, who resigned in January after eight years to take a job with a consulting company.
The group's president, Barry Lamb, said DeOrsay "will build upon the projects developed over our 25-year history." These include water quality monitoring, advocacy against overdevelopment, plans for waterfront trails and educational programs.
DeOrsay, executive director of the whaling museum for more than a decade, said, the bay and harbor "are such valuable and fragile resources, bringing so much to the quality of life we enjoy."
But, he added, "We have only to look around the Sound to appreciate that this healthy and beautiful estuary is under constant threat."
Lamb and DeOrsay said their first objective is to build the organization's financial capacity to achieve its mission.
With the approval last year by local governments of an Action Plan for the Oyster Bay Watershed, Lamb said, "We need to strengthen our base of support and raise the funds needed to translate the plan into action."
DeOrsay, a lifelong sailor and Coast Guard licensed captain, has been the skipper of many tall ships.
At the whaling museum, DeOrsay said, he was in charge of six full-time employees and doubled membership revenue.
From 1994 to 2001, he was vice president for operations at the Independence Seaport Museum in Philadelphia.