No doubt about it. President Reagan was trifling with the duck-is-a-duck rule ("if it walks like a duck and quacks like a duck, it must be . . .") when he talked about the armada we have sent to Central American waters.

He has sent aircraft carriers, a battleship, countless support craft and thousands of men on "routine" maneuvers. But he insists that it's silly and perverse to think of this as warlike.

Consider, he said, that for every dollar of military aid we are sending three dollars in economic aid, which proves that the great armada has to be there on a mission of peace. The president's explanation, which did not entirely tally with that of his aides, has been scornfully received.

That's a natural reaction, considering the prevalent belief that in roughly similar situations other presidents have deceived us.

But scorn is not a thoughtful reaction, nor a sufficient one. As usual, the president's critics and interrogators are thinking only one move ahead and are pressing for an explicitness in U.S. Central American policy that no one could conceivably wish for.

The United States began to lose its finesse in these matters that spring day in 1960 when President Eisenhower, having first denied it, ill- advisedly admitted that, yes, we were sending U2s over the Soviet Union. No one doubted that this was so and long had been. But nations of large responsibilities must do some things that cannot be acknowledged without diplomatic offense, among them the aerial surveillance of another sovereign country.

Eisenhower's "honesty" was widely hailed, but it wrecked a summit meeting and may thus have laid the groundwork for the dangerous missile crisis of 1962. The point is, it has gradually become an article of belief that not even the slightest deception or ambiguity in the deployment of U.S. arms, men, operational intelligence capabilities, etc., can be tolerated.

By this exalted standard, there could have been no Pax Britannica, for there could have been no screen of ambiguity and deception for the small army and navy that maintained order over three-quarters of the world's land surface. "Perfidious Albion" would not have been allowed--by Parliament--to be perfidious.

I don't know whether it will be an effective move or not to station a bit of naval muscle in the waters off Central America. It probably depends on the result, which in turn depends on what conclusions about that deployment are drawn by the arms suppliers and the Nicaraguans. But the effect of the move will certainly be null if President Reagan cannot create a bit of suspense, if he must now take a pledge of official pacifism to satisfy Congress and the armchair strategists of the press.

The questions must be heard to be believed: Mr. President, doesn't it bring war closer to send warships down there? Mr. President, if we're aiding the anti-Sandinista rebels in Nicaragua, why don't we do it openly?

Suppose the president answered: "Yes, we're expecting a shooting war down there-- sent the ships to provoke it, as a matter of fact." Or, "Yes, although we have diplomatic relations with the government of Nicaragua, we really are trying to overthrow it. Remind me to tell the ambassador to let the Sandinistas know that next time he visits with them."

If such answers are not sought--and how could they be?--why are such questions asked? A little healthy curiosity never hurts, of course. But there is clearly the thought of so narrowing the president's discretion in the exercise of power and diplomacy that he couldn't send a rowboat to Miami without congressional permission.

That would leave us a presidency shaped to the most Walter-Mitty-like dreams of congressional supremacy. But it would be hell on what is left of the Pax Americana.