"Holy mackerel!" someone aptly exclaims as the man from Atlantis races a porpoise the full length of a swimming pool and wins. Being a friend to animals and a speaker of the whale's own gaelic, he gives the fish reward to the runner-up.

"The man from Atlantis," a four-installment mini-series that is a cross between "Flipper" and "The $6 Million Man," premieres at 9 o-clock tonight on Channel 4. NBC is obviously toying with the idea of making this a fall series, which sort of makes sense from the network that replaces a flop show about an invisible man with another flop show about an invisible man.

"Atlantis" makes an agreeable enough low-key adventure, but director Lee H. Katzin and writer Mayo Simon seem under the delusion that they have something other than a cartoon strip on their hands. They tend to play for poignance rather than adventure; tender scenes between the water-breathing hero and the women doctor who finds him washed up like a beached grunion, however, are strictly for snickers.

Still, after about an hour of splashing around for a plot, things pick up with the threat of yet another world conquest by yet another mad scientist, this one a tubby salt water lives under the sea and recruits scientists with his phantom submarine. The man from Atlantis swims into his lair, one thing leads to another, and the world squeaks by again.

This could have been a lot more fun, but "Atlantis" suffers from the indecisiveness anf restraint which mars so much of junk television; the show seems lost somewhere between affable trash and futile aspirations to respectability.

Victor Buono does about as much as he can as the villian of the piece, Belinda Montgomery is pleasant as friendly Dr. Merril, and as the man from Atlantis himself, Patrick Duffy certainly communicates the guy's uncommunicativeness. He wears contact lens that make his eyes look googly and is required to spen so much time in the water that he probably emerged with a case of dishpan body.

But then, you have to expect to suffer for your art.