JOHN J. SIRICA has only one regret about the way he handled the Watergate cases that he tried. Sirica says now that he wishes he had allowed broadcasts of the White House tapes President Nixon turned over to the Watergate special prosecutor so that the American people could hear for themselves how Nixon participated in the Watergate coverup.

Other than that, Sirica says, he is at peace with himself, relaxed, able to enjoy life now that he has assumed the semiretired status of a senior United States District Court Judge. Sirica's corner office on the second floor of the U.S. courthouse here, a constitutional nerve center during the critical days of Watergate, had settled into a monastic quietude until about a month ago. Then the tempo picked up again.

Sirica's Watergate book, "To Set the Record Straight," is to be published April 24.

Always a man who enjoyed talking, Sirica at 75 remains vibrant and alert. He has made a remarkable recovery from the near-fatal heart attack he suffered in 1976. He jabs you in the stomach from time to time to emphasize a point, occasionally referring to some Watergate culprit in terms that federal judges do not use while sitting on the bench. Despite the celebrity that has come to him in the last six years, he still has that strange air of innocence and wonderment about him, the third genration Italian-American who has become famous - and now probably wealthy - beyond his wildest dreams.

In his book, Sirica says that Nixon should have been indicted-rather than pardoned-for his role in the Watergate conspiracy. And, had Nixon been convicted in his court, Sirica says, the former president would have received a jail sentence.

"I still have that lingering feeling that no matter how great his personal loss, Nixon did manage to keep himself above the law," Sirica said in his book.

"He was forced to give up his office, but he was not treated the same way as the other defendants. His associates served time in jail. He received a large government pension, and retired to his lovely home in San Clemente.

"I think people still wonder whether the concept of equal justice under law really applies if one climbs high enough in terms of wealth, power, or influence. . .

It still bothers me that Richard Nixon escaped that equal treatment. I feel that if he had been convicted in my court, I would have sent him to jail."

Stepping out from behind the restraints imposed when a judge is sitting on the bench, Sirica offers his candid views on the Watergate cases he tried and the lawyers involved.

The propriety of a federal judge's writing a book about a case which he tried is itself a matter of controversy. Some lawyers already have criticized Sirica for producing the book.

"I don't agree with them," he said in a recent conversation. He ticked off five other judges, "off the top of my head." He said they all have written books. Whether his view that judges should speak out-"as long as it doesn't interfere with judicial activities"-prevails or not doesn't seem to bother him. "Oh, I'll get criticized, but what the hell," he said.

That statement, letting the chips fall where they may, comes near to the heart of the matter when one gets around to assessing Sirica's role in Watergate. Had it not been for that scandal, he would certainly have been the self-described "obscure judge" he writes about in his book.

Sirica will not go down in history as being a judge like Oliver Wendell Holmes or Learned Hand or Louis Brandeis, who are remembered for the profundity and eloquence of their opinions. In fact, aside from his Watergate decisions, Sirica's opinions are eminently forgettable. Prior to Watergate, his reputation was mediocre. More frequently reversed on appeal than judges like to be, Sirica had an inflexibility about him, a narrowness of perspective and a simpleness of attitude-coupled with a bad temper-that suggested to some that he was not well suited for the bench.

He had in fact gotten the job because of Republican party connections rather than for any reputation as a top-notch lawyer or first-rate legal mind. His high school education in Washington was spotty. He never went to college. He dropped out of Georgetown University Law School twice before finally finishing, and then thought for a while that he might do better to be a professional boxer.

Exercising the discretion granted him under federal law as chief federal judge for the District of Columbia, Sirica assigned himself to the Watergate break-in case. He says he wanted the case because he wanted to get to the bottom of it. His flaws suddenly became strengths. The other side of inflexibility is immunity to pressure. Narrowness of perspective can also be single-mindedness. Simpleness can also be calrity as in devotion to basic principles.

That was the side of Sirica's character revealed in Watergate. In his own halting, sometimes inarticulate way he expressed a rustic devotion to certain fundamental ideals that appealed to somethimg basic in the American character. During the period when the Nixon White House was deceiving, disembling and stonewalling, Sirica was insisting that the full, true story of Watergate come out in public.

He was criticized for becoming a prosecutor in black robes. In his book, Sirica quotes with obvious approval Circuit Court Judge Harold L. Leventhal's opinion upholding the conviction of the two defendants in the Watergate break-in case to the effect that "the precepts of fair trial and judicial objectivity do not require a judge to be inert." Sirica puts it more bluntly. "Simply stated, I had no intention of sitting on the bench like a nincompoop and watching the parade go by," he says.

Sirica's were the homely virtues of determination and perseverance. He remains, as he was during Watergate, a man conscious of his limitations. "Let's face it," he said in a conversation last week. "I'm not an intellectual. I know that. I do think the most important thing a person can possess is common sense. Common sense can carry you a long way."

One irony of Watergate is that while so many well educated lawyers of sterling reputation failed so miserably in meeting their obligation to uphold the law, this "obscure" federal judge of limited education and plain manner rose to the occasion.

People looking for individual heroes in Watergate miss the point of it all, according to Sirica. Watergate was a tragedy all right, but it also was a triumph of American democracy. Sirica understands well why he became a hero. It was not simply because he was personally courageous. Rather, he became a folk hero because his conduct symbolized in a pure, unadulterated, straightforward way the ideals we profess to believe in but blush to assert simply, but boldly, as he did-truth, justice and equality before the law.