The National Sea Rescue Institute raised the alarm over captain John Matambu and his passenger after they didn't return to Sodwana Bay on South Africa's northeastern coast on Saturday evening having gone out early that morning.
The charred wreck of Matambu's speed boat, Magnum Too, was discovered by a search plane on Sunday in the Dobela area of Mozambique, some 170 kilometers (105 miles) up the coast.
“Both men remain missing in unknown circumstances,” the National Sea Rescue Institute said.
Matambu's cousin told local media that a petrol canister and a medical kit were found near the boat.
The NSRI said some of the circumstances “appear to indicate that foul play may be involved.”
The man who hired Matambu to take him out on the 21-foot (6.4-meter) boat apparently gave false contact details, NSRI spokesperson Craig Lambinon said. The booking was made for a man, his wife and a child, but only the man boarded the boat, Lambinon said.
"Hopes are that this has all been a misunderstanding, and foul play was not involved, but at this stage all possibilities are being examined," Lambinon told national broadcaster SABC. “The fact that the boat has been found across the border is obviously the difficulty now.”
The NSRI received a report that the boat was spotted at 10 p.m. Saturday night heading north toward Mozambican waters, it said, the last sighting before its charred hull was discovered the following day.
The Police Sea Borderline Control agency and the Police Search and Rescue unit are also involved in the search. Authorities described Matambu as a well-known, experienced and respected boat captain in Sodwana Bay.