The Maldives military says it stopped seven tourists it claims were sailing to a highly-guarded US military base in the Indian Ocean.
The German and Italian tourists and 16 members of a fishing boat's crew were intercepted after leaving the Maldives.
One minister said they were sailing to Diego Garcia, a British-controlled atoll that hosts a US military base.
One of the tourists said the claims they were going to Diego Garcia were "absolutely ridiculous".
Giorgio Rosi Belliere, an Italian businessman on board the boat, said they were on a whale watching tour and were not travelling to the atoll, some 750km (470 miles) away.
"We have done nothing wrong," he told the Maldives' Minivan News.
The Maldives Minister for Defence, Mohammed Zuhair, said the crew had failed to comply with international and local law on leaving the Maldives.
The Haveeru Online news site said the crew had not brought their passports, even though they were in international waters.
There are conflicting reports as to whether those on board were arrested. Minivan News said they had all now been barred from leaving the Maldives.
Diego Garcia is a highly-guarded military base once used for bombing raids on Iraq and Afghanistan.
In January, a former senior official in the George W Bush administration said the US government carried out interrogations of terror suspects at the base.