Bill Brener and the "Chum King" have worked the waters at the mouth of the Potomac River long enough to win grudging respect from a few local charter skippers, who might wave to him or grunt a hello.

But the 25-foot Chum King remains a "turkey boat," which is what charter captains call all private craft that show up weekends and follow the pros shamelessly to the best fishing holes.

A turkey boater's crowning achievement is to turn the tables and have the professionals follow him to a hot spot. That had happened to Brener exactly once in 20 years before this summer. But now he has a new, expensive gadget and suddenly he occasionally feels the glare of eyes upon him.

It's a nice feeling.

The gadget is an expensive, television-style monitor that depicts in technicolor detail the marine life and the bay bottom under the boat. It's so effective it probably should be illegal.

I first saw one last winter aboard an offshore clamming boat out of Chincoteague, Va. The captain flipped the switch when we hit the clam grounds 20 miles out to sea and pointed to the multicolored line depicting the bottom 120 feet down.

"See that?" he said, indicating a yellow splotch. "That's a clam."

Brener's command of the machinery is a little less complete, basically because he hasn't read the directions. But even in the early stages of familiarity, he can spot a pile of what appears to be sea trout or bluefish, freeze the frame, zoom in and start catching.

"What got me," said Brener, who runs a janitorial company in Arlington when he isn't fishing, "was the ad in Saltwater Sportsman that said you could literally see bait, like shrimp, rising up from the bottom.

"With my old flasher (depth finder), I'd think I saw something but I couldn't focus in on what I thought I saw. You're looking around, trying not to bump into other boats. I would think I saw something, but I'd be chumming up nothing."

Brener invited me to try the toy last week on a day that turned out unfit to fish. We went, anyway, into the teeth of a 20-knot nor'wester.

After a brief, unsuccessful stop at the mouth of the St. Mary's River, he headed for Cornfield Harbor, which lay protected in the lee of Point Lookout State Park. On the way, in the turbulent chop of the main river, Brener spied seagulls wheeling and caterwauling below dark clouds. "Bluefish breaking," he said, and rigged up shiny lures.

The birds, using their own natural device, could look down upon terrorized schools of menhaden stuck between the devil and the deep blue. The baitfish were in double trouble, victims of shrieking gulls that scooped them off the surface if they rose or ravenous bluefish that gobbled them up below if they didn't. We tossed our offerings into the frenzy and caught a bundle.

But Brener had eyes elsewhere.

"I see some charter boats out by the point," he said. "Maybe they're catching sea trout."

He stopped en route to buy one soft crab from a crabber, which was all the fellow had. Farther on, four or five charter boats were in a tight pod. We spied an angler boating a pink-gold trout of about 10 pounds.

Brener circled and studied his machine, which showed barren bottom 20 feet down. Suddenly, he popped out of his chair with a shout. "There they are!" The freeze-frame on the monitor showed a six-foot hill of fish piled off the bottom.

He dunked a soft-crab bait overboard and was fast to a 10-pounder.

Unfortunately, the bait ran out. Brener went off in search of more and by the time we returned, the trout had scattered. We set off looking.

Around and around he went in widening circles, until several hundred yards from the initial strike he spied another hill of fish and hopped down from his seat. We put the lines out. Fish on!

As he went for the net, Brener looked up to see the turkey-boater's dream. Charter boats, heading his way.

The evolution of the depth finder has reached such technological perfection that I asked Brener what the next step might be.

"If they make one any better," he said, "when you turn it on it will say, 'Destroy me.' "