Maputo city council said in a media statement that the incident occurred late on Wednesday, and among the missing are two men and two women.
“The group departed from the city of Maputo on a boat from the Nautical School and took a break for meals on the Island of Inhaca.
“The missing are part of a group of eight South African tourists”, according to the statement.
Teams with representatives of several government entities are involved in the searches, which a helicopter belonging to a private individual has joined.
With its picture-postcard Indian Ocean beaches, the Island off Mozambique’s seaside capital has un-spoilt coral reefs and laid-back southern African charm.
Mozambique has been slowly putting its civil war past behind it and emerging as an attractive, albeit slightly chaotic, tourist destination.