Two men, a Singaporean and a German, have been arrested for trespassing by the Malaysian authorities after they were found at Middle Rocks, near Pedra Branca.
The duo said they had been diving in the Singapore Strait on Saturday, but the tide brought them to Middle Rocks, Malaysia's maritime outpost, at about 8pm.
They have been detained in Johor for investigation.
The Malaysian Maritime Enforcement Agency's director-general, Datuk Seri Ahmad Puzi Ab Kahar, said yesterday: "They said it was for leisure but strayed and floated (into Malaysian territory). We're trying to verify their statements with the embassies and diving company."
Mr Bjoern Cochlovius, 49, and Mr Anjashmara Mohd Khalid, a 29- year-old diving instructor, had started their dive at South Ledge on Saturday afternoon. They told the authorities they deviated from their route by accident.
Mr Cochlovius is a doctor and professor of immunology based in Singapore, according to his social media profile.
The Malaysian authorities said a Bintan resort boat tried to take the men after their arrest, but were refused, pending investigation. Mr Ahmad Puzi said: "We need to ascertain that there's no threat."
He added that diving in the area was a "very high risk", especially at night, given the busy sea traffic.
Officials added that it was unclear why the duo had chosen to dive in that particular area after dusk. The men did not have permission to dive in Malaysian territory.
In 2008, Singapore was awarded ownership of Pedra Branca by the International Court of Justice (ICJ). The size of a football field, it lies north of two other islets - Middle Rocks and South Ledge. The ICJ ruled that Malaysia's claim over Middle Rocks stood, but made no definitive ruling over South Ledge, which is visible only at low tide.
The area was the focus of a territorial dispute between Singapore and Malaysia, which began in 1979 after a map published by the Malaysian side showed that Pedra Branca fell under its territory.