For the past month, it has been difficult to hold a political conversation in France without the subject of la piscine -- the swimming pool -- cropping up.

The newspapers have been full of pictures of Defense Minister Charles Hernu getting into, or sometimes out of, a swimming pool. The cartoonists have had a field day with drawings of frogmen swimming about in a tub of water marked piscine. There have been sly references to "watergaffe" and President Francois Mitterrand taking a swim.

La piscine, as every Frenchman knows by now, is journalistic slang for France's not-so-secret secret service, which is embroiled in one of the biggest scandals of its scandal-strewn history: allegations that it was involved in the sabotage of a Greenpeace ship in New Zealand. The nickname derives from a public swimming pool not far from the Paris headquarters of the General Directorate for External Security (DGSE), the French equivalent of the Central Intelligence Agency.

The arrest of two officers of the secret service in New Zealand on charges of murder -- a photographer was killed in the July 10 explosion -- is simply the latest in a long string of murky goings-on at the "swimming pool." During the past four decades, the French secret services, under a variety of names, have acquired a reputation for dirty tricks and political skulduggery.

Although an official French inquiry tentatively exonerated the General Directorate of wrongdoing in the "Greenpeace affair" this week, Prime Minister Laurent Fabius has conceded publicly that the agency continues to suffer from serious "shortcomings." In a statement Tuesday, he called for greater administrative and parliamentary controls over the secret service.

Any major reforms are likely to be resisted by senior intelligence operatives who seem bewildered by all the fuss over the sinking of an environmentalists' ship on the other side of the globe. In a newspaper interview today, the former head of French counterespionage, Roger Wybot, said that the government would do better to let the Rainbow Warrior affair simply "blow over."

"Look, this isn't the first time that different French governments have had to conduct this kind of operation. Whatever their political coloring, they have all taken this road," he said.

The revelations that all is not well at la piscine have proved a political embarrassment for the ruling Socialist Party, which came into office in May 1981 promising to clean the place up. Known in previous incarnations as the SDECE, the BCRA and the DGER, the agency was regarded by many senior Socialists as a bastion of right-wing views.

According to the authors of a recent book entitled "La Piscine," Pascal Krop and Roger Faligot, there was practically a revolt in the service's Action Division when the Socialists came to power in alliance with the Communists. Agents were told by their superiors to practice a "scorched earth" policy to prevent the most sensitive files from falling into the hands of France's new left-wing rulers.

Krop and Faligot say resistance to the Socialists was strongest among the officers of the Action Division's training center for combat frogmen in Corsica: the headquarters of four of the five French agents named by New Zealand police as suspects in the sinking of the Rainbow Warrior. Here, according to the authors, officers refused to hang up Mitterrand's portrait for several months after he replaced conservative president Valery Giscard d'Estaing.

During the past four decades, both right and left have made use of the "swimming pool" to embarrass political enemies and check up on presumed friends. A wartime precursor of the General Directorate, the Central Bureau of Intelligence and Action, took it upon itself to rid the Free French movement in London of potential rivals to the late general Charles de Gaulle.

In the 1950s, the agency known as SDECE was used by a succession of governments to infiltrate and control independence movements in Indochina and Algeria. Recruitment policies were lax: the SDECE's "honorable correspondents," as its part-time agents are known, included gangsters, released criminals and mercenaries.

There is convincing evidence that the SDECE carried out a series of murders in Algeria among supporters of the pro-independence National Liberation Front. One of its most spectacular coups was the arrest of the front's leader, Ahmed Ben Bella, in 1956 after forcing his plane to land in Algiers.

But the SDECE's biggest scandal was unquestionably the kidnaping, and presumed murder, of Moroccan opposition leader Mehdi Ben Barka in October 1965. A left-wing nationalist who had fallen out with the Moroccan king, Ben Barka was seized in broad daylight in the center of Paris while waiting for an appointment with de Gaulle.

Organized by SDECE agents who also worked for the Moroccan authorities, the Ben Barka affair infuriated de Gaulle. It led to a thorough shake-up in the service, the dismissal of some of its wilder operatives and the resignation of its director, Gen. Paul Jacquier. The agency, which previously had been under the control of the prime minister, was incorporated into the Defense Ministry.

By most accounts, the SDECE became considerably more professional in the 1970s. Its new director, an urbane aristocrat named Count Alexandre de Marenches, succeeded in putting a stop to the more questionable "dirty tricks" and free-lance activities. Discipline in the agency, with 2,000 employes, was tightened up.

Under Giscard's presidency, the Action Division was used for a series of operations in Africa, including the recruitment of mercenaries to fight in Chad and the toppling of emperor Jean-Bedel Bokassa in Central Africa.

Vehemently anti-Soviet, de Marenches refused to continue as director of SDECE when Mitterrand appointed four Communist ministers to his Cabinet. His place was taken by Pierre Marion, a friend of Defense Minister Hernu.

Marion lasted less than two years at the agency, which he rechristened the DGSE, a signal of his determination to introduce reforms. The appointment was apparently a disaster. Old-timers in the secret service resented what they regarded as his heavy-handed interference. Morale plunged, as did the quality of information provided by the directorate.

According to French officials, Mitterrand was dismayed by the puerile nature of much of the agency's reporting, particularly on the Soviet Union. The president was also embarrassed last year when -- on the basis of incorrect information from the agency -- the government announced that all Libyan troops had left Chad, only to be publicly contradicted by U.S. satellite photographs a couple of days later.

Marion's replacement, Adm. Pierre Lacoste, has the reputation of being a patriotic, somewhat reserved career officer unlikely to take independent political initiatives. According to the official inquiry into the "Greenpeace affair," the decision to send the agents to New Zealand was taken by him following consultations with both Hernu and Gen. Jean Saulnier, Mitterrand's senior military aide.

In its written instructions to the directorate, the government ordered it to collect information on Greenpeace to "anticipate" the organization's future actions. In French, the verb anticiper, which was underlined twice in the document, also carries the connotation of "to prevent" or "to forestall."

Greenpeace, which in the past has sailed ships into the French nuclear testing zone, had planned to send a flotilla into the zone this summer.

As plausible a scenario as any of events leading up to the sinking of the Rainbow Warrior, Greenpeace's flagship, was provided by Jean Daniel, a friend of Mitterrand and editor of the left-wing Nouvel Observateur. In an editorial this week, he speculated that the decision to sabotage the ship could have been taken out of "an excess of zeal" by some middle-ranking official in the intelligence services. Daniel said the solid consensus in France on the need to develop the country's independent nuclear deterrent, or force de frappe, had created a climate of opinion in which opponents of French nuclear tests in the Pacific were regarded as national enemies. Ambiguous orders could be misinterpreted easily or hardened up as they made their way down the bureaucratic hierarchy.

"I imagine that the little chief who made the decision to blow up the ship isn't very intelligent. But I do not exclude that he was a patriot. He wanted to teach France's enemies a lesson . . . . Like John le Carre's novels, that's the way it happens," Daniel wrote.