By NANCY SAN MARTIN

  In a sign that the Cuban government is taking rumors about a mass exodus seriously, radio and television broadcasters spent Wednesday issuing advisories warning the population not to fall victim to ''false reports'' that boaters from South Florida would be traveling by sea today to pick up anyone who wanted to flee the island.

  The government's communique, broadcast throughout the day, said that any unauthorized boat entering Cuban waters could be seized and its occupants could be
  charged with migrant trafficking.

  The Cuban government also blamed radio broadcasts out of Miami for inciting a potential mass exodus, though it did not point to any one particular source.

  ''Counterrevolutionary radio stations have been broadcasting the rumor that on July 4 boats from the United States would come to pick up people who want to travel illegally to the United States,'' the government communique stated. ``These are vulgar provocations by the terrorist mafia in Miami. . . . Nobody will be authorized to leave the country illegally.''

  The rumors, which appear to have a strong following in Havana Province and Pinar del Rio, began circulating last week following a speech by President Fidel Castro in which he warned that migration accords could be dissolved if the United States and the U.S. Interests Section in Havana continued to participate in acts considered hostile.

  The government has since said Castro's words were misinterpreted.

  ''It wasn't a threat, it was simply a factual analysis,'' said Luis Fernández, a spokesman for the Cuban Interests Section in Washington. ``It was an honest reflection of what is occurring.''

  Cuba watchers in the United States have accused Castro of setting the stage to unleash another exodus like those in 1965, 1980 and 1994 as a way to rid the island of alleged malcontents.

  Whatever the intent of Castro's speech, word on Cuban streets about a pending exodus spread quickly and included instructions for those who wanted to leave: Wait just outside the 12-mile limit that marks Cuban waters.