A commercial fishing boat carrying 300 Cuban refugees radioed a distress call Friday night that it was sinking 25 miles north of Cuba, a Coast Guard spokesman said.

As of 12:30 a.m. today, two Coast Guard cutters, the Vigorous and the Courageous, were unable to find the vessel in the 5- to 10-foot seas, spokesman Richard Barrett said.

The incident, which followed by a day the deaths of three refugees by asphyxiation in another crowded boat, came as the Coast Guard moved to choke off traffic to Cuba and many Cuban-Americans continued to be openly defiant of President Carter's cutoff of the Freedom Flotilla.

Meanwhile, the scene in Key West was one of despair and defiance. Sweating in the midday heat Friday, an old man crouched on the floor of his grimy little boat, struggling to fix the motor. His chest was bandaged -- a broken rib, he said -- and there was a look of desperation in his eyes.

Before President Carter's efforts to shut down the Cuban's Freedom Flotilla, "the hope had been building inside me," he said. "Now I am crushed." But the man, Tomas Rodriguez, 70, a Miami electrician, was getting ready to go to Cuba nonetheless.

"We have made so much sacrifice to buy our boats. We have lost our jobs. We have lost everything. Now we can't wait any longer. Our relatives in Cuba are dying of hunger."

All along the docks of this sweltering tropical city Friday, the tales of anguish were the same. Cuban-Americans have spent their life savings on boats and supplies to bring brothers, sons, mothers and husbands out of Cuba. Now they face threats of prison terms and fines of $1,000 per refugee if they try it.

The administration plan to have boats in Mariel return empty is apparently not working. Refugees continued to stream into Key West Friday in record number. By 9:30 p.m., 4,000 had arrived in the previous 18 hours, bringing the total to 52,000 in three weeks.

U.S. Coast Guard officers began boarding all vessels headed south from Key West Friday in a stepped-up effort to cut off the boatlift. Rear Adm. Benedict Stabile, commander of the Seventh Coast Guard district, announced the new policy in a news conference in Miami.

A spokesman for his office said later that two of the first boats boarded turned back to Key West after being requested to do so. Cmdr. Samuel Dennis said Coast Guardsmen "had to remove the crew" and take custody of one boat.

U.S. skippers face $50,000 fines and 10 years in prison for violating the ban on bringing in Cubans illegally.

Stabile also announced that he is bringing major reinforcements to the south Florida area to assist the increased law enforcement effort. Six cutters, nine planes, six smaller boats and 14 boat safety detachments will join the Coast Guard vessels already deployed in the Florida Straits, he said.

But the threats that the government will prosecute Americans who bring back refugees -- while they are widely regarded largely as bluff -- are causing hundred of individual tragedies for families caught up in the diplomatic standoff between Carter and Cuban President Fidel Castro.

Some 20,000 Cubans, whose American relatives requested their departure, are waiting to leave on the flotilla, having lost their jobs, homes and ration cards when they applied for visas. They have been branded outcasts for wanting to emigrate, and have no means of survival in Cuba if their U.S. relatives abandon them.

Rodriguez fears for his son and a grandson he has never met. They are waiting for him in an outside enclosure in Mariel, sleeping on the ground with no food, he said.

Rodriguez' daughter-in-law, Marta, who was helping him prepare the boat, said they had paid $14,000 for the 32-foot outboard. "We have spent all our savings," she said.

"We were planning to buy a house. Now, forget it. Everything we have done is for nothing. My aunt has called from Cuba. She says, 'Where are you?'"

If her father-in-law and husband, Roberto, go to Cuba, "I'm afraid they'll be arrested when they get back," she said. "We love this country, but we don't know what to do."

More than a dozen little boats were lined up at the marina, and loud arguments in Spanish could be heard up and down the dock between those afraid to leave and those determined to rescue their families.

"They won't put you in jail -- that's peanuts!" Roberto Ramas angrily exclaimed to Marta Rodriguez. "I'm leaving anyway," he said.

Ramas, who owns a small exterminating company in Manhattan, spent $7,000 on a boat that fell off its trailer and was badly damaged. Friday, two other boats offered him passage to Cuba.

Since Castro won't agree to a U.S. government-sponsored sealift, "ain't nobody going to help us but ourselves," said Ramas. "I've made a lot of sacrifices, and all I get from the U.S. government is promises."

Equally defiant was Julio Mendino, 37, a Charlotte, N.C., mechanic who was helping organize some 30 boats ready to leave for Cuba. Mendino had just arrived from Mariel with a cargo of 14 refugees, and Cuban officials had promised he could fetch his relatives on a return trip.

The president's order "doesn't stop nobody," he said. "I'm not scared of the fines. My family is waiting for me. They were put out in the street. Their electricity was cut off. They lost their jobs. People are throwing stones at them. They are in danger."

But a few boats away, Ramon Mota, 51, a Miami mechanic, sat dispondent. "I have a wife and five children," he said. "I can't afford to go to prison. President Carter is a family man, too. He is religious and humanitarian. Maybe he will change his mind."