The FBI has said it found no evidence an explosion on board the Maldives president's boat was caused by a bomb, a report said, raising questions about his deputy's arrest over the incident.

President Abdulla Yameen was unharmed in the blast on his speed boat which authorities described as an assassination attempt. The September 28 explosion left his wife and two others slightly injured.

Police arrested vice president Ahmed Adeeb nearly a month later for treason as he returned to the Indian Ocean archipelago from an official trip abroad.

The arrest followed a series of sackings of government officials including Yameen's defence minister, which have fuelled concerns of political instability and further damaged the Maldives' image as an upmarket tourist destination.

After a review of evidence, the FBI found that debris from the blast, first considered possible remnants of a bomb, were in fact parts of the boat, The Wall Street Journal reported on Saturday.

"There is no conclusive evidence to attribute the explosion on the boat to an IED," a Federal Bureau of Investigation spokesman told the journal.

The US agency, which had been asked to assist in the investigation, had examined the scene of the blast and chemical trace testing, it said.

The Maldives has faced tough international criticism over the jailing in March of its first democratically elected leader Mohamed Nasheed, after a rushed trial that the UN said was seriously flawed.

Yameen, who came to power in November 2013 following a controversial election, faces international censure over his crackdown on political dissent, including on supporters of opposition leader Nasheed.

(AFP)