"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.