Boat people fleeing poverty in the Dominican Republic are boarding flimsy motor boats and braving a hazardous 90-mile stretch of the Caribbean to reach Puerto Rico, gateway to the United States.

Many of the illegal aliens drown during the three-day voyage. Most who make it ashore later pose as Puerto Ricans to slip past immigration officials and board flights to New York.U.S. Coast Guard and Border Patrol officials in Puerto Rico say they don't have enough manpower to halt the exodus, which has increased sharply in recent months as the Dominican economy worsens.

The Dominican Republic has a population of 6 million, and as many as 30,000 try to leave each year, officials estimate. They say no more than 10 percent to 20 percent of Dominicans illegally entering Puerto Rico are caught. Some 3,000 people were detained in 1988. The rest either make it or drown.

Authorities complain that the problem isn't getting the attention it deserves because it's happening so far from the U.S. mainland. Puerto Rico, a U.S. commonwealth, is 1,000 miles southeast of Miami.

"There are human beings dying, and they also pose an economic threat to the United States; but the consciousness of this is zero," said Capt. William A. Caster, head of the Coast Guard's Greater Antilles Command.

U.S. officials say the Dominican exodus has reached the scale of Haiti's in 1985, when thousands of boat people were intercepted by the Coast Guard and hundreds of others washed up on Florida's shores.

"The only difference is that Haitians were dying off the U.S. coast," said Caster, who also led the Haitian operation.

Another difference is that the Coast Guard has an agreement with Haiti allowing the Coast Guard to board Haitian boats to check for illegal aliens. As a result, some 4,000 Haitians were arrested in 1988.

There is no such accord with the Dominican government, although talks are to start in the new year about signing one.

"We are focusing on the drug problem, not aliens," said Caster. "We don't go out looking for Dominicans. We're coming across them on our patrols and when we do, it invariably turns out to be a rescue operation."

Facing 30 percent unemployment rate and 60 percent inflation, Dominicans are lured to the United States in search of jobs and to Puerto Rico because Spanish is spoken here and Puerto Ricans are U.S. citizens. A Dominican who can pass for a Puerto Rican doesn't need documents or a passport to fly to New York, where most illegal Dominicans end up.

Authorities concede it is easy for Dominicans to get past immigration officials at San Juan's airport with a well-rehearsed "New York" when asked where they're from.

"There aren't that many differences between the languages and cultures" of the Dominican Republic and Puerto Rico, said James Walker, the district director of the Immigration and Naturalization Service. He said more than 1 million Dominicans are in the United States, most of them illegally.