Each night since last Thursday, once the airwaves clear of daily traffic, the message from the U.S. Coast Guard to the hundreds of American boats here begins to blast over the radio: "Captains, leave Mariel immediately with your ships empty and return to Florida."

The first day the message was broadcast, a number of boats followed the orders and set out for Key West without the Cuban refugees they had come for.The exodus lasted about 10 hours. Then, according to one boat captain here, "the Cuban government caught on to what was happening."

Since then, Cuban gunboats, with sailors on deck carrying automatic weapons, have turned back those who try to leave with empty ships. "There hasn't been any violence," a Florida shrimp boat captain who has been waiting here nearly two weeks said. "But if somebody points an AK47 in my face and tells me to turn around, I know what that means."

Some Cubans in Havana have suggested that those boat captains arriving with full boats in Key West find it convenient to say they were forced into accepting passengers despite the U.S. ban on the boatlift. But while rumors circulate in Mariel, the stories of the gunboats unfold.

None of the U.S. visitors questioned here could verify rumors that those who chose to pass the gunboats empty are charged $25,000 by the Cuban government. But as the shrimp boat captain said, "The U.S. Coast Guard says I'll be fined $50,000 if I turn up in Key West with my boat full of refugees. It's just a question of who I want to pay. I came here to take people out and I'm staying."

There have been no reported arrivals of new boats in the past two days and Havana newspapers now say the number of boats in Mariel has been reduced to 854. The newspapers also note, however, that by government count, 5,639 refugees departed Sunday and that few of those boats waiting for passengers appear disposed to leave without them.

When asked about stories that their boats are being overloaded with passengers by the Cubans and sent off to Florida, the captains said the situation varies with each departure.

"A bunch of us were charted by Cubans living in the U.S. to come down and pick up relatives," said a captain from Florida. "The way it works is, when you get here, you give the police a list of names of people you want, and they tell you they'll notify your boat when everyone's ready."

In Havana, many registration points have been set up where residents who want to leave fill out forms and, in many cases, declare themselves to be common criminals, homosexuals, prostitutes or simply antirevolutionaries.

It is believed that these declarations facilitate one's departure by allowing authorities to keep up the fiction that all who want to go are social misfits.

After signing up, the registrants -- now self-declared undesirables -- go home for what is often a nightmarish wait, particularly in working-class neighborhoods. In what one Western diplomat in Havana describes as a process similar to the Chinese Cultural Revolution, citizens were reportedly advised through their neighborhood organizations to harass the registrants, although they were warned against physical violence.

One observer in Havana interprets the harassment as a national psychological deception, designed to convince those who are staying that the emigres, rather than leaving by choice, are being driven out by Cuba's true patriots.

Although many of those leaving are in fact convicted criminals or political and social misfits, many disconcerted neighborhoods have seen the most strident revolutionaries on the block pick up and disappear, along with government officials, professionals and technicians.

When a boat's turn comes up -- within days or sometimes weeks of arrival -- police arrive at the homes of those who have registered and at the dwellings of those whose names have been brought by the outsiders. Families normally have a half-hour to gather their belongings and get on a bus taking them to Mariel.

Mixed in with the families are those the government has taken out of local prisons and other institutions, or those simply told on their blocks that they must go.

The refugees are loaded directly on the boats in Mariel and the captains generally have little say over who or how many get on board.

"One buddy of mine left only half full yesterday," a Florida captain said Sunday. "But others get too many people put on board."

Still, the boat captains interviewed said that those boats in danger of sinking during the crossing are generally older craft that 'haven't had so much weight on them in 25 years." But the Cuban government allows no choice as to the numbers or identities of the passengers.

However, the captains interviewed denied that the boats were forcibly sent out in bad weather. On Saturday morning they said, a growing storm resulted in Cuba stopping all departures temporarily. In any case, they can listen to U.S. weather reports on their ship radios they said.

But the waiting and the lack of control over their situations, has brought the visitors, emotions to a point that belies the almost somnambulant calmness of Mariel a small industrial city 25 miles west of Havana.

In the shadow of huge, constantly hissing smokestacks of the hydroelectric plant that provides power for the capital, the wide natural harbor is filled with large Cuban and U.S. fishing vessels, smaller powerboats and aging Florida shrimpers and crab boats.

Downtown where Cuban residents stroll along dusty sunbaked streets, there is no evidence of the visitors. Those who come to pick up passengers are restricted to a small waterfront area filled with newly opened supply and souvenir shops, along with two cafes that deal only in greenbacks.

One can purchase a case of Cuban beer for $20 with a $20 bottle deposit, a newly minted souvenir T-shirt printed with the slogan, "Mariel Brigade 1980," or for $300, a fighting cock.

The cocks are brought aboard the waiting ships by Cuban exiles to amuse themselves while waiting for their passengers to be delivered.

For those who stay aboard their boats, a gabage ship picks up refuse for $1 a day, floating fruit and dry goods boats pass by daily, and at night there is the diversions of the "Pinares" -- a floating Cuban restaurant and bar that the visitors have dubbed "the love boat."

Those who want to leave Mariel can pay $15 for a bus ride to the Triton Hotel in West Havana. The visitors are not permitted to go anywhere else, however, and the Triton, like Mariel itself, is officially closed to Cubans and the press and guarded by police and Army troops.