AT TIMES the Georgetown waterfront fight has seemed destined to drag on forever - so nearly an agreement is encouraging. And there is much to applaud in the pact signed last week by Interior Secretary Cecil D. Andrus, City Planning Director James O. Gibson, National Capital Planning Commission Director David M. Childs and the private developers involved. It spells out the terms for limited commerical and residential development in a 5.9-acre triangle below K Street and east of Wisconsin Avenue. The rest of the waterfront, including a 160-foot-wide strip in front of the new complex, is finally to be transformed into a 17-plus-acre park.

The fact that Sen. Charles McC. Mathias Jr. (R-Md.) presided over the treaty signing is noteworthy. A while ago, some of the Georgetown's antidevelopment militants persuaded Mr Mathias to try to stop any re-building on the entire slope between the C & O Canal and the Potomac. On second thought, Mr. Mathias realized how expensive, excessive and arbitrary that would be, and joined the search for a sensible compromise.

The Georgetown die-hards have now enlisted another lawmaker, Sen. Mark Hatfield (R-Ore.), to carry their flag. He has introduced a bill directing the Interior Department to buy the "development triangle" and include it in the waterfront park. That could cost the federal government more than $25 million, and would deprive the District of substantial tax revenues - and for what? A wider park with more of the Whitehurst Freeway in clear view.

Yes, lower Goergetown is congested - and it's going to get worse. But the fate of the "triangle" won't make crucial difference. While it may not be perfect, the proposed development is not so massive, ugly or otherwise horrible that Congress should intervene. Continuing this fray will just retard the work that everyone supports: cleaning up the rest of the waterfront and addressing the traffic problems that will persist in any case. That's where the energies of federal and city agencies, and concerned lawmakers and citizens, should be directed now. The real disappointment would come if, two or five years from now, private development has advanced - and public officials are still scraping around for funds or arguing over where to move the city's parking lots.