After years of talking about it, Fidel Castro and the world's foremost underwater explorer, Jacques Cousteau, finally have set a date to do some deep-sea diving together. The coinventor of the self-contained underwater breathing apparatus (alias scuba) and Cuba's hard-line communist leader will spend parts of October and November together off the coast of Cuba when Cousteau takes his operation into the Caribbean for four months this fall.

Cousteau, who says Castro "loves diving" and for years has been inviting him to Cuba, also says Haiti's "Baby Doc" Duvalier has been trying to get him to do the same thing off Haiti. At a French Embassy dinner, Cousteau sounded uncertain about whether Duvalier and he will become diving partners, though not about Haiti, where he is also heading for the first time.

The dinner, given by Ambassador Emmanuel de Margerie and his wife Helene, climaxed a day in which Cousteau was one of 12 recipients of the Presidential Medal of Freedom at the White House. Cousteau's White House connection goes back to President Reagan's California days, when he told Reagan he'd be president someday. Reagan, a former lifeguard who swims like a lifeguard should, but apparently hasn't asked to go diving with Cousteau, had the Frenchman to lunch shortly after he moved into the White House.

Cousteau's film on the illegal drug industry along the Amazon River, "Snowstorm in the Jungle," was shown in Atlanta last month when Nancy Reagan escorted wives of 15 foreign leaders to an international drug abuse conference there. He thinks a conversation his son, Jean Michel, had with aides of Mrs. Reagan last winter may have triggered her White House conference.

Not so impressionable, however, is Danielle Mitterrand. Cousteau told of going live on French television last month to introduce his documentary. Noting that France ranks second in cocaine abuse, he said he felt somebody in the French government had to take on the responsibility to change that.

"I was asking for contributions to send 11,000 cassettes of the film to universities and schools, and I said Mrs. Reagan had invited the first ladies of most countries to Washington to talk about the drug abuse problem, but that France had not responded," Cousteau said.

"Five minutes later the telephone rang and it was Madame Mitterrand asking me to come to the Elyse'e Palace. I went there and she began to give me reasons why she didn't go to the conference." Cousteau went on, "I said, 'It's no use to tell me that. Why don't you come with me on television and say it?' She said, 'Well, I'm too shy to speak on television.' " What Cousteau isn't telling are Danielle Mitterrand's reasons for not joining Nancy Reagan and the other first ladies.

Said he: "That's the part I can't tell you."