If a TV series called "Potomac Patrol" ever sprung forth, it would be fantasy, sure as death or taxes. For no producer could handle river reality.

The premiere would doubtless show cops chasing flotillas of smugglers off the shores of Alexandria. In the next episode, we'd have to watch bank robbers swimming to Rosslyn -- and freedom. Maybe we'd even get a runaway ocean liner homing in on a Georgetown boathouse owned by a Charlie's Angel.

But for the real Metropolitan policemen who patrol the real Potomac and Anacostia Rivers, life is more like... well, life.

Crime on the river tends to be skinny dipping. One arrest a year is considered a lot. Most of the time, river police duty is methodical, uneventful -- and a little chilly.

Especially in winter. Between December and March, most of the pleasureboaters of July are hiding in the bottom of a landlocked bottle of cognac.

That makes winter on the rivers a special sort of experience, and an especially satisfying beat for a policeman. Because the rivers are virtually deserted, an easy kind of slowness descends. Keeping the peace tends to mean keeping the silence. It is a job, in that sense, much like the one the old cop-on-the-beat used to have.

The one drawback in winter is that the Floating Fuzz can sometimes get lonely. So perhaps it was understandable that officers Kevin Doheny and Mike Chukinas warmly welcomed a visitor who bundled up and came aboard to witness a recent morning of patrolling.

"If it weren't for you," Doheny explained, "it'd just be us and the trees."

This was not some idle observation about the foliage along the shore. It was nautical nuts and bolts.

It had rained steadily the previous three days. Any time that happens, whole trees get uprooted upstream. Looking like a migrating flock of alligators, they start floating down the Potomac toward the Chesapeake Bay.

This bleak winter day, about one tree a minute was floating downstream. Each had enough of a head of steam to seriously damage -- and maybe overturn -- a small boat.

But Doheny, at the wheel of his 28-foot police cabin cruiser, neatly avoided them all. And that was about all he had to worry about all morning. The navigating demanded constant attention, but as Doheny noted, "It beats chasing muggers."

The Harbor Section of the Metropolitan Police has always left the muggers to others. Formed 98 years ago, it is one of the oldest nautical police agencies in the country. Since the turn of the century, "The Harbor" has been responsible for 43 miles of river -- the Potomac from Alexandria to Little Falls and the Anacostia from the District-Maryland line to Hains Point.

The 26 men assigned to the Harbor Section are among the most highly trained policemen in the world. Five are fulltime scuba divers. Another three conduct free classes in boating safety for the public. Every one of the 26 is an expert swimmer and must have previous nautical experience -- usually in the Navy or the Coast Guard -- to qualify for assignment to The Harbor.

As Lt. Thomas McGlynn puts it, "Once you get a man down here, they don't leave."

McGlynn, a 24-year police veteran, not only commands the harbor squad, but also serves as master of Washington's harbor. As harbormaster, "I issue building permits, anchorage permits, you name it. Anything administrative having to do with the river, I'm the man," McGlynn said.

McGlynn is also a man whose fingers are tightly crossed.

The last two winters, the Potomac and Anacostia "grew" 14 inches of surface ice for much of January and February -- and "The Harbor" ran itself ragged trying to keep hikers, fishermen and pranksters off it. This year, however, the two rivers have not frozen over, and after mid-March, the weather is expected to warm up enough to make ice virtually impossible. For McGlynn, March can't come too soon.

Last winter was especially difficult. Some college students decided to beat the January blahs by pushing a Volkswagen onto the Potomac under Key Bridge (amazingly, it didn't fall through). And throughout February, Sunday strollers forsook the C&O Canal to tramp across the middle of the river.

No one fell in, but it wouldn't have been hard when the ice was only 14 inches thick. That much ice looks like enough to support a human, but McGlynn cautions that it really isn't.

"If you fell in, your chances of survival in water temperatures like we get in the winter would be nil -- about four or five minutes," he said.

In the fiscal year that ended last October, the Harbor Section rescued 519 persons and assisted 222 boats. It also had the grisly responsibility of recovering 28 dead bodies -- most of them summer drowning victims.

"The river is much cleaner in the winter. You can even see the bottom in some places," said Doheny, as he motored under the 14th Street Bridge. "And as far as bodies go, the winter is easier. You not only don't get as many -- but in the winter, a lot of times what you think is a body will be a turtle or a tire or a tree trunk."

Ill will between sailors and motoroboaters remains a major problem on the Potomac and Anacostia, according to McGlynn. "They both think they have the right-of-way," he explained. And novice boaters who don't know the locations of sandbars "will always be a problem."

But for shoot-'em-up action, the TV types will have to look elsewhere. "We're like fire extinguishers," said McGlynn. "We're not exciting, but you've got to have us."

And in the winter, except for floating trees, the men of The Harbor are about all the dark, lapping rivers have.