OF COURSE, we're all sophisticated, and we chuckle heartily at stories of sea monsters sighted in the Potomac. This is the same Washington, after all, that, in "The Day the Earth Stood Still," stood still and chortled as Patricia Neal and Michael Rennie begged us to mend our ways. We didn't listen then, yet we survived. And so presumably, we will not listen now, even though James W. Dutton of Maryland and 20 assorted Virginia residents have seen with their own eyes evidence of serpent - like creatures, or the creatures themselves, sporting in our river.

Besides, we have the word of James E. Douglas to bolster our confidence. Mr. Douglas is chairman of the Potomac River Fisheries Commission, and also of the Virginia Marine Resources Commission, and he announced last week that whatever monster was spotted "obviously belongs to the ecosystem." Mr. Douglas pursued this put-down by going on to call the monster "nothing more than a big snake." A big snake - naturally.

Who in his right mind would believe otherwise? Is there anyone among us who actually imagines a crazy reptile like Godzilla (if Godzilla was a reptile) rising from the sea like a great green cloud and heading for the capital with murder in his heart? God knows, we have enough day-to-day nuclear-age monsters in this city as it is - big grey ones like Commerce and the Post Office. We don't need to go overboard to scare up a scare.

So don't trouble your pretty head about sea serpents, Washington. Just toss your cocktail parties, serve your canapes, gossip your gossip. And think not a moment of the possibility that somewhere out in the dark river, like giant aggies side by side, lie two eyes, watching.