THE SLICED BREAD syndrome has overtaken the law of the sea. You know--one side, to whip up enthusiasm for a new idea, says it's the greatest thing since sliced bread. In that spirit, well-meaning people started suggesting in the 1960s that codifying the rule of law at sea and providing for the international sharing of the "common heritage" of seabed resources would build a kingdom of earthly harmony and bring us all closer to the God of our choice.

Inevitably, the vision faded. What was left was 1) a treaty compromised to a point where reasonable people can differ on its merits and 2) a school of criticism claiming that the law of the sea is the worst idea since sliced bread. That school is currently in its glory, daily decrying the treaty as a sellout to third world tyranny, a betrayal of the free enterprise system, the biggest boondoggle in the history of the world and so forth.

Fortunately, these cries are nonsense. A more sober look reveals a treaty which, in its time, the Reagan administration has substantially improved. Mr. Reagan's negotiators have won, for instance, the guarantees he sought of Western corporate access to strategic seabed minerals. These go on top of the assurances of maritime passage secured in the Carter years. But predictably, the lobbyists have objections. So do the ideologues. They are on the prowl, hoping to persuade a somewhat distracted administration to devour its own child.

That brings us to Friday, when the treaty, to which 150-odd nations have devoted eight years, comes up for adoption in New York. How the United States responds will depend in some measure on substantive negotiations that may not be concluded until literally the last moment. At that point, the American choice will be either to permit --quietly--adoption of the treaty by consensus, or to make a sour statement or even to force a vote in which the United States alone will be opposed.

The liveliness of the Washington play makes it uncertain what the administration will do. Here is what it should do. It should give itself the benefit of its doubts. Rather than take a gratuitous slap at an extraordinary negotiating process or cut itself off by a negative vote, it shouly andld let the treaty be adopted by consensus and use the six months until signing-- that's the important event--to think it over. Our suspicion is that, as it focuses, it will be pleased by its work.