MIAMI (AP) _ Vice President George Bush, in Miami to swear in six members of the Radio Marti advisory board, took a spin in a U.S. Customs speedboat and said his South Florida Task Force is tightening the noose on drug smugglers.

Radio Marti, President Reagan's project to broadcast news and music to Cuba, went on the air seven months ago. It has offices in Washington, D.C., and a transmitter in the Florida Keys.

''We see Radio Marti as the voice of truth in Cuba,'' Bush said during his visit Monday. ''Castro's response to Radio Marti shows how dangerous truth can be to a dictator.''

The day the broadcasts started, Cuban President Fidel Castro halted an agreement to take back convicted criminals who had arrived in the United States during the 1980 Mariel boatlift. Only about 200 people had been returned to the communist island nation by then.

Asked if the United States planned TV broadcasts that would reach Cuba, Bush said, ''I have no comment on that, but the more we can broadcast to Cuba, the better.''

Earlier, Bush took the wheel of Blue Thunder, a 39-foot catamaran, for a 20-minute ride around Biscayne Bay. The $150,000 Blue Thunder is Customs' newest weapon against narcotics trafficking.

''I'm here today to see another way in which we're tightening the noose on drug smugglers,'' Bush said at Customs headquarters after his ride.

''Drug smugglers have been using top-of-the-line speed boats. Soon we will have boats - the Blue Thunder boats - that can match anything they have on the water.''

Customs officials said Blue Thunder was paid for with money from the sale of boats confiscated from drug smugglers. But the catamaran has not been directly involved in any drug-related arrests since Customs began using it six months ago, officials said.

''We're gaining in the war on drugs,'' Bush said. ''You have to keep this in perspective. We are in this as a marathon, not a sprint.''

In the nearly four years since the Vice President's Drug Task Force came to South Florida, drug agents have made about 10,000 arrests and consfiscated record amounts of narcotics.

But officials say the amount of drugs entering the United States through South Florida is still increasing and that the street price for cocaine, which was about $60,000 a kilo in 1982, has decreased to $30,000 currently.

Sworn in by Bush to serve on the 10-member Radio Marti board were: Midge Decter, executive director for the Committee for a Free World; Joseph Glennon, a retired foreign service officer; Antonio Navarro, senior vice president of W.R. Grace and Co.; Jose Luis Rodriguez, chairman of Corky Foods Corp. and president of M&R Farms; Dan Sawyer Jr., vice president for R.R. Donnelley & Sons Co. of Chicago, and John Silber, president of Boston University.

Jorge Mas is chairman of the board and Stu Sweet is its executive director.

Two positions remain unfilled.