The islands in Preah Sihanouk province are part of a new national marine park, the Koh Rong Sea National Park.
Experts say region on cusp of an energy revolution as photovoltaic prices plummet.
Sam Rainsy added that Japan had strong leverage over Cambodia due to its considerable development assistance and investment.
The train service will temporarily be free of charge for passengers until August.
Hun Sen ordered those involved in the plot arrested and for the police to step up security in the capital.
The World Khmer Conference participants also demanded the immediate release of Kem Sokha and other political prisoners and called for free and fair elections.
A ruling Cambodian People’s Party spokesman said Sam Rainsy’s appeal to voters was “in vain”, adding that turnout was expected at between 60 and 70 percent.
Prime Minister Hun Sen on Monday blessed the Cambodian people during a Khmer New Year ceremony, declaring his victory over the opposition party.
Long Sokunthy, 42, represents some 300 people who are locked in a long-running land dispute with a local tycoon.
Foreign Minister Taro Kono said that Tokyo would help Cambodia with free and fair elections, according to a spokesman for his office, Norio Maruyama, who confirmed that Japan would supply ballot boxes for the July polls.
Cambodia National Rescue Party (CNRP) was dissolved by Supreme Court last November at request of PM Hun Sen's government, which alleged it was plotting to take power with the help of US.
Local nongovernmental groups also invited to monitor July vote ruling party is expected to win because main opposition party was dissolved.