Three men surprised the couple, identified as 60-year-old Ronald Francois Wollbeck and his wife Maria Rawi, 69, early yesterday on their boat in San Marcos bay near Sao Luis do Maranhao.
An argument ensued and Wollbeck was shot in the chest, according to the reports.
The victim's wife was quoted as telling the G1 news site that the assailants fled in a small boat.
The couple had been in Brazil since December 21 and arrived Saturday in Sao Luis, the capital of the state of Maranhao, from Recife, police said.