He was 59.
Roundfield's wife apparently had some trouble while swimming off of Baby Beach, and Roundfield tried to assist her, the Aruba Herald reported. Witnesses saw his wife return to shore, but they said Roundfield was taken out to sea.
Authorities found Roundfield's body under some rocks, the Herald reported.
Roundfield's survivors include his wife, Bernie, sons Corey and Christopher, and their grandchildren.
Roundfield played 12 seasons in the NBA from 1975 through 1987, playing for Indiana, Atlanta, Detroit and Washington.
Besides being named to the All-Star team three times, Roundtree was named to the NBA all-defensive team five times, and was a member of the all-NBA second team in 1980.
Statistically, his best seasons were 1981-82, when he averaged 18.6 points an 11.8 rebounds, and 1982-83, when he averaged 19.0 points and 11.4 boards.
"Danny was the most honest and upfront person I knew, and I'm very stunned at hearing the news of his death," former Hawks teammate Dominque Wilkins, now a Hawks vice president, said in a statement released by the team. "Danny's one of my closest friends and he was a tremendous influence on my NBA career, on and off the court. He taught me how to be a professional and took me under his wing. My thoughts and prayers go out to his family, I will truly miss him."