"You know, we're up against a potential disaster here." Elizabeth Merrill dares to admit on "Man From Atlantis" tonight. The NBC show, making its bow as a weekly series at 9 o'clock on Channel 4, rises to most of that potential.

Yet it's kept afloat here and there by Patrick Duffy in the title role of ex-Altantan Mark Harris - the most engaging aquatic acrobat since Flipper - and by Victor, Buono, the fat poet of the "Tonight" show, as the effetely villainous Mr. Schubert. whose opening prank is to try melting the polar ice caps with microwaves.

When this fiendish scheme sinks, Buono pouts in disappointment. "I wanted to see the tops of high-rises peekabooin' out of the waves," he says. You sort of like him for that.

But the kidnapping of Atlantan Harris, who can breathe underwater and whose built-in [WORD ILLEGIBLE] turns microwaves to so much much. Naturally he escapes. As for the attractive Merrill, her role in this adventure is mainly as anchorwoman; as watches the gill guy go hippity-bop on her submarine TV screen and shouts out, "Mark - wait a minute! We'll send out a dialectic conducer!"

There is nothing wrong with "Man From Atlantis" that a quick exile to a Saturday morning time slot wouldn't cure. NBC hs a peculiar devotion to silly berves in prime time - as in last season's "Gemini Man" disappearing act and "The Invisible Man," with invisible ratings, before that. But the network doesn't give these shows enough adult appeal. Bueno's tongue-in-cheek mugging is only a token effort in this direction.

Kids may be impressed, though, by the hero's googley eyes, by special effects max. Gene Warren's "Sea Wars" submarines and underwater explosions, and even perhaps by that dialectic conducer. Week after week of Buono and Duffy playing catfish-and-mouse, however, should soon wear thinner than water, Watch for a crash dive.