Williamson, who was 55, was managing director and portfolio manager of hedged U.S. equities strategy at Chilton Investment Co in Stamford, Connecticut. He joined the firm in January 2011.
He was staying with family members at a house on Figure Eight Island, a gated development on a barrier island near Wilmington.
Williamson was reported missing Sunday morning after he did not return from a party Saturday night that took place at another location on the island, less than a mile from the house, according to New Hanover County Sheriff Deputy Jason Augst.
After a search that included a helicopter and a dive team, Williamson's car was discovered underwater off a boat ramp. Authorities do not suspect foul play, Augst said.
Williamson was one of six portfolio managers at Chilton, which manages a total of about $6 billion in assets. Richard Chilton Jr., the firm's chairman and chief executive officer, will take over management of Williamson's U.S. Hedged Equities fund.
"Robert was a dear friend, and he will be missed," a statement from Chilton read.
"His curiosity, enthusiasm, intellect, and passion for stock picking were gifts he shared with all who knew him. We extend our heartfelt condolences to his family at this extremely sad time."
Williamson joined Chilton after a hedge fund he co-founded with Edward McAree folded at the end of 2010. Before starting his own fund, he spent two years at his uncle's firm, Tiger Management Corp. He began his career at Donaldson, Lufkin & Jenrette.
Williamson was also the step-brother of Louis Bacon, the founder of Moore Capital Management.
(Reporting by Emily Flitter)