COMING SOON to a river or large body of water near you: The Floating Cinema! !

It is the brainchild of Jon Rubin, who teaches film at the State University of New York at Purchase. So far this summer it has been shown on Cayuga Lake, the Erie Canal and on Saturday night the two-screen cinema "vaudeville" will be shown at 8 on the Potomac between Key Bridge and the Kennedy Center.

The idea for the Floating Cinema came to Rubin, 35, out of his experiments with putting film in different settings. "I'd been doing a lot of thigs that involved changing the context of cinema; taking film outside movie theaters and putting it in places where people had no expectations of seeing it."

Four years ago, an incident pushed him toward finding a way to put his films in a "totally unexpected" context. While a "low-budget tourist" in the jungles of Guatemala he woke up in the middle of the night to find "a procession of icons on floats going by, carried on people's shoulders.

"It was totally unexpected," he adds. "There was no electricity in the place. Their lights were powered by generators being pushed in wheelbarrows. And the ceremony was a bizarre mix of Indian and Christian culture."

It took him a couple of years, but finally two summers ago he rounded up money from the usual suspects: the National Endowment for the Arts, the Ford Foundation and the Jerome Foundation, and set off down the Allegheny and Ohio Rivers. "We followed the river charts and found towns where the setting was right. We'd come in at dusk, set up without announcement, do the show and leave.

"We would do the entire performance without explanation or hype. So people had no help figuring it out."

Although his work is far from mainstream, he says it is not comparable to the films of noted avantgardists like Stan Brakhage and Hollis Frampton. "My work is more literal...One of the problems of the avant-garde is its barriers of exclusivity. This was an attempt to have an open event. They [the people in the river towns] weren't even aware they had walked into it."

The Floating Cinema is set up on two lightweight pontoons, each with its own self-contained projection and sound systems. There is a six-man crew, including Rubin. Two people are on each barge, one to steer and the other to run the projection equipment.Rubin stays ashore to oversee the performance and one other person scurries around taking care of odds and ends, such as Rubin's 5-month-old daughter.

They travel from place to place in a single pontoon trailer carefully modified to fit the two barges. Rubin has a truck to pull it from water to water and for storage of related equipment.

The show Saturday is a collage piece. "It has a lot of different short pieces," he says. It is not abstract. "I'm involved in recognizable images. It's kind of a vaudeville show. A lot of shorts stuck together."

Rubin doesn't always have time to inspect the sites of his performance. "We wing it sometimes for economic reasons." In the case of Washington he has had a chance to check out the Potomac.

But he is not sure about the response he will get here. He knows that many of those who watch will turn out because of the efforts of the AFI organizers, and he's afraid they will create expectations in the minds of the audience. Experience has taught him that an audience with expectations gets fidgety if a film is different than it has anticipated. "After five minutes it's hard to get them to sit still."

He hopes that some people will just stumble upon the show, because then he gets the response he wants. "A couple of nights ago we did an impromptu performance on the Erie Barge Canal. After the show about 20 people followed us from the road in their cars as we floated down the river. They wouldn't let go."