GEORGE BUSH pledged in his '88 campaign that there would be "no net loss" of wetlands under his presidency. That sounded unequivocal, but equivocating over wetlands has become a Washington pastime.In 1989 the federal agencies that oversee wetlands protection - including the Environmental Protection Agency and Army Corps of Engineers - published a manual intended to serve as the single federal guideline for wetlands designation. Before, the various agencies had their own definitions. This manual drew sharp criticism from business and agricultural interests that felt it would unduly expand protected wetlands acreage in the United States. Initially, misunderstanding arose over whether farmland was exempted under the guidelines - in fact it was. Still, the perception remained - and not just among disgruntled developers - that the definition of wetlands was being stretched too far. The controversy reached a peak last summer when the Council on Competitiveness, chaired by Vice President Quayle, pushed for revisions in the guidelines to address the concerns of business. Scientists from the various federal agencies have completed field research to assess the impact of the 1989 manual as revised. The research showed, according to news reports, a substantial reduction in protected wetlands under the revised version. It also showed, according to analysis by the Environmental Defense Fund, that the original '89 manual would only minimally increase the amount of designated wetlands over an earlier manual published by the Corps of Engineers in 1987. But the question of how much bona fide wetland exists and deserves to be protected is still open. Whose criteria should prevail? The scientists who drew up the '89 manual emphasized such generally agreed on factors as the types of vegetation and soil. How much weight should the objections of landowners and industry be given? Over the next couple of months these questions will be wrestled with, and Washington will settle on a manual to guide wetlands protection in the US. The job of designating wetlands should be done thoughtfully. The interests of private landowners ought to be considered. But government's dominant purpose - as Mr. Bush's pledge seemed to recognize - is to preserve the water-purifying, flood- control, and wildlife benefits of our shrinking wetlands.