The tourists' Guatemalan tour guide also spoke by mobile phone with the radio station.
"We don't know where we are but we've been moving about in the boat for several hours," Mauricio Dubon said.
"We haven't been physically hurt, but we haven't eaten since 10 o'clock [in the morning] and they are moving the boat to different canals off the Rio Dulce."
The farmers who seized the Belgian tourists on Friday belong to the same group that captured 29 Guatemalan police last month in the Caribbean coastal town of Livingston.
The police were held for almost two days before being releasing in exchange for talks about legalising the farmers' land claims and dropping charges against Choc, a community leader who supporters say is fighting for land rights.
Choc was arrested on February 14 on charges of illegal land invasion, robbery and holding people against their will.
Close to half of Guatemala's population are indigenous peasants, many of them landless, who often invade land for subsistence farming.
Land disputes were one of the catalysts for Guatemala's 1960-96 civil war between leftist guerrillas and the government, which left around 250,000 people dead or missing.
Alvaro Colom, the country's president since January, has promised to reduce poverty and violence but has so far faced problems in controlling crime.