To a mad genius like David Milch, the phrase surf’s up has very little to do with water.

With Milch’s Western, “Deadwood,” riding off into the sunset, his new HBO project, “John From Cincinnati,” is about a seriously whacked surfing family in California, the Yosts.

A mysterious stranger (Austin Nichols) – who may or may not be from Cincinnati – shows up chez Yost looking for surfing lessons. Drama ensues. HBO has ordered 12 episodes, to launch this summer.

Milch calls it “surf noir,” but this particular surf is unlike any we’ve seen before, at least on this planet.

John “is about the effort to identify the genuine coordinates of reality,” Milch says. (A recovering heroin addict, alcoholic and compulsive gambler, Milch is familiar with that effort.)

“It’s such a strange idea. The strangeness of it is its essence. To try and demystify it is probably to do it a disservice. To fix the coordinates of the reality is itself the dramatic structure.”

Got that?

The cast includes Bruce Greenwood, Rebecca De Mornay, Ed O’Neill and Luis Guzman. Luke Perry, former “Beverly Hills, 90210” stud (and surfer!), joined last month.

Originally, Perry was to have appeared only in the pilot. When his new NBC show, “Windfall,” got blown away, he went full-time. It’s Perry’s second HBO series; he had a recurring role on “Oz” in ’01-’02.

“Surfers, more than most people, have difficulty identifying reality,” says Milch, 61, whose credits include “NYPD Blue” and “Hill Street Blues.”

Not that Milch is an expert in riding the waves. “I probably know about as much about surfing as any Buffalonian.”

Lucky for Milch that his co-executive producer/writer is Kem Nunn, author of such dark surf-themed novels as “Tijuana Straits” and “Tapping the Source,” and a writer/producer for “Deadwood.”

Milch is no John-come-lately. He wrote the original script about five years ago. Titled “John From Elsewhere,” it had the same elements as Cincinnati “John,” Milch says, minus the surfing.

“It was about mistaken impressions; an uncertainty about where this guy’s from.”

HBO passed, and Milch began “Deadwood.” A year ago, Milch says, HBO asked him to help develop a series combining the premise of “Elsewhere” with the Fletchers, a real-life surfing family around whom HBO was going to develop a show.

“It was not exactly my cup of tea,” Milch recalls. “If it were a story that began and ended with surfing, I probably would have felt I wasn’t the guy to be involved in it.”

As for John’s geographical origins, “someone makes a mistaken inference” that he hails from Cincinnati, Milch says.

“Cincinnati does not leap to mind as a hotbed of surfing. If we had to bet – and we do, if we’re compulsive gamblers – he’s probably not from Cincinnati.

“Sometimes people have mistaken impressions about the nature of reality. I continuously have mistaken impressions about the nature of reality.”

Back to “Deadwood,” shooting is set to begin in June or July on two, two-hour HBO movies that will wrap up the series, according to Milch.

After HBO had nixed another full “Deadwood” season, Milch was “quite ambivalent” about tying up myriad storylines in just four hours. “But we live in the world of the possible. I’m happy with the compromise.”

To accommodate the show’s compressed length, each episode will represent several years in the life of the lawless mining town instead of one day, Milch says.

Juggling two TV movies and a new series would be taxing for somebody with all his marbles, so we can only imagine how Milch is holding up. “That sound you hear is broken crockery,” he says.

Locking in the cast will be tough, too. Some players – stars Ian McShane and Timothy Olyphant, for starters – are essential. “We’ll do what we have to do to reach a critical mass of availability,” Milch says.

“As Samuel Goldwyn once said, ‘If necessary, we’ll cast Mexicans.’ That’s show biz.”

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If NBC’s “Today” adds a fourth hour, “Today” chief Jim Bell says he could see Kelly Ripa of “Live With Regis and Kelly” co-hosting it.

“We might reach out to her,” Bell says. “It would be foolish not to consider somebody who’s an established presence in the morning. She’d be a logical fit for the back end of the show.”

No word on Ripa’s contract situation. A rep for the syndicated “Live” says: “We never comment on contractual matters.”

Billy Bush is a more likely candidate. Bell likes him, his deal with NBC Universal’s syndicated “Access Hollywood” is up in the spring, and he filled in as “Today” co-host Dec. 11-13.

“We’re looking at everybody, and not just for the fourth hour,” Bell says. “There’s a constant evaluation of talent, in-house and out-of-house. Billy’s in-house, so of course we’ll take a look at him.”

Odds are 50/50, Bell estimates, that “Today” will add a fourth hour.

NBC suits are expected to make a decision by early ’07, possibly next month, Bell says. If “Today” tacks on a 10 a.m. hour, the show would air from 7 to 11 a.m. EST.

Most likely, it would run as two distinct, two-hour shows. Matt Lauer and Meredith Vieira would stay with the 7-to-9 a.m. mother ship. Ann Curry and Al Roker, now sticking around for the 9 a.m. hour, may stay until 11.