By JACK ANDERSON

WASHINGTON - Cuban Premier Fidel Castro, himself the intended victim of U.S. assassination attempts, plotted in 1963 to send frogmen on a commando raid against President Richard Nixon's vacation home at Key Biscayne.

The secret plan called for the underwater raiders to blow up the winter White House. It is unknown whether Castro expected them to catch the American President on the premises and kill him in the explosion.

The only hint of the plot appeared afterward in a cryptic State Department announcement that a Cuban official at the United Nations, named Lazaro Eddy Espinosa Bonet, had been asked to leave the country. The statement explained tersely that his expulsion was related to the "security of the office of the President."

From intelligence sources, we have now learned the suppressed details. It should be emphasized that the raid was a contingency plan and that no known attempt was made to implement it.

HOWEVER, Espinosa Bonet tried to recruit a Cuban-American servant inside the Nixon compound to bug the President's seaside villa and to draw up blueprints of the security arrangements.

To coerce the servant, the Castro agent threatened him and his family in Cuba with physical harm. The frightened servant was told that he would be provided with microtransmitters, which he should plant throughout the Nixon home. These would have to be monitored by Cuban agents in Miami or aboard fishing boats off the Florida coast.

The requested blueprints would have been used to guide the attacking frogmen. Our intelligence sources speculated that the raiders would have sought first to blow up the communications complex inside the Nixon compound. Then, presumably, they would have struck the Nixon home itself.

Despite the threats; the servant refused to betray Nixon and informed U.S. authorities of the plot. The servant's story, though vague on some details, was convincing enough for the State Department to expel Espinosa Bonet.

If Castro would consider sending frogmen against Nixon, some investigators wonder whether he might have instigated the assassination of President John F. Kennedy. We first raised this possibility on March 3, 1967.

WE HAD learned that the CIA, using Mafia killers, had tried to eliminate Fidel Castro. This was at the time, we reported, that Robert Kennedy "was riding hard on the agency." We questioned whether Kennedy "may have approved an assassination plot, which then possibly backfired against his late brother."

Former aides of the late Robert Kennedy have now confided that he was furious over our column. It was published at a time of strained relations between Kennedy and then-President Lyndon Johnson. Kennedy thought Johnson had leaked the story to us.

The aides recall, significantly, Kennedy acknowledged to them that our story was correct about the assassination plot, but he denied that he had approved the plot: he told them, on the contrary, that he had stopped it.

IT TOOK us until January 1971 to get the details of the CIA attempts upon Castro's life - details that were denied at the time but were confirmed last year by the Senate Intelligence Committee.

In a series of columns we told how the CIA had recruited Mafia mobsters who made six attempts to kill Castro. We named the mobsters and their CIA contacts. And again, we raised the possibility that Castro had retaliated against John F. Kennedy because of the CIA plot upon his own fife.

Significantly, the attempts to knock off Castro continued through 1963 until he angrily warned on Sept. 7, 1963, that he was aware of the CIA's efforts, that two could play the same game and that U.S. leaders would no longer be safe. Two months later, President Kennedy was gunned down in Dallas.

LAST WEEK, CBS in a two-hour television documentary stressed the theory that Castro had retaliated against Kennedy. Sen. Robert Morgan (D., N.C.), who played an active role for 18 months in the Senate investigation of the CIA-Castro caper, was quoted.

I believe the circumstances in this case are so strong," he said, "that they convince me beyond every reasonable doubt that the assassination of our President was an act of retaliation for what we had tried to do in eliminating Castro."

The Cuban leader responded with a vigorous denial. "It would have been absolute insanity," he declared. The CBS moderator Bill Moyers carefully termed the retaliation theory as "unsubstantiated speculation."

WE HAVE also called attention in past columns to conflicting evidence that anti-Castro Cubans might have been involved with the accused assassin, Lee Harvey Oswald, in an attempt to make Castro appear guilty.

Now the plot to use frogmen against Nixon adds fuel to the speculation and another bizarre chapter to the stranger-than-fiction story of Cuban-American relations.