SOLOMONS ISLAND, MD. -- For years, Mike Sullivan has been a familiar sight around Maryland waters in his fast, twin-diesel charter boat, Miss Dolly. You never knew where he might turn up as he took Dolly where the fish were. He even had T-shirts made up advertising his penchant for "fishing the entire Chesapeake Bay."

But while Dolly was a queen of the charter boat fleet, eventually she wasn't queenly enough for the changing Chesapeake, and this year the nomadic Sullivan came up with something bigger and twice as fast.

Miss Dolly had four-cylinder engines; the new Dolly Diesel has twin turbo V8s, supercharged to 375 horsepower each. Miss Dolly was 38 feet long and carried six; Dolly Diesel is 47 feet long and carries 15. With a top speed of 38 mph, she races to the fishing holes in half the time of her older sister, guzzling up to 25 gallons of fuel an hour along the way.

When Dolly Diesel roars across the Chesapeake, throwing spray like a fireboat; even the dourest old watermen turn their heads to admire her.

Sullivan, said one rival charterman, has raised bay fishing to a new level. "It's a whole different world," said Robbie Robinson, whose lovely bay-built Miss Regina looks matronly by comparison.

The big question, of course, is why? Given the state of the Chesapeake, in sad decline over the last 20 years as it chokes on the side effects of overdevelopment, sewage effluent and changing farm practices, why build a $125,000 fishing boat to meander a waterway the fish are abandoning?

The answer is that the bay ain't dead yet, and Sullivan, as president of the Maryland Charter Boat Operators' Association, isn't ready to give it up for dead.

So he sat down last year and reasoned out his troubles: There were still plenty of people left who wanted to fish, he decided, and plenty of fish if you could get to the right spots, which were increasingly distant.

"There's still a terrific resource from Cove Point {near Solomons} on south," Sullivan said. "The problem is, the business is still in the north."

Sullivan's base for years has been Chesapeake Beach, a 45-minute drive from Washington. He liked it there, but with Miss Dolly and her limited range he was in a trap. If he stayed close to population centers, he risked disappointing his customers with poor catches in the declining waters of the middle bay. If he tied up at some far-flung port like Crisfield or Hooper's Island, he risked not getting the customers at all.

So Sullivan took the $125,000 plunge and hired Gene Travers in Trappe, Md., to build him the hottest charter boat on the bay. He took delivery in July.

These days with Dolly Diesel, Sullivan can get from Chesapeake Beach to the rich and untroubled waters of Hooper Straits on the lower Eastern Shore in an hour, running hard at about 25 mph. Once there, he figures he can produce a satisfactory catch of trout, spot, hardhead, flounder and blues 95 percent of the time and still get his parties home for supper.

Or, he can tie up a little further south at Solomons and make the run to the fishing grounds in 40 minutes.

And what a ride it is.

Last week, Sullivan ran some charter skipper friends across for a demonstration. From the moment the big Caterpillar diesels roared to life outside the Tiki Bar on the main drag, you knew you were in for something.

There is a peculiar sound a big, powerful marine diesel has as it's clicked into gear, like the sound of a prison door slamming. Ker-klunk!

Dolly Diesel is so wide at 14 1/2 feet that she barely fits in her slip. "You need to grease her just to get her in and out," said Calvin Tyler as Dolly creaked out of the slip at prodigious idle speed, then burbled out to the mouth of the Patuxent, where a decent sea was building.

When Sullivan put the throttles up, Dolly Diesel's bow fairly leaped out of the water and spray went flying over the windshields. He flipped on the radar to make sure nothing unexpected "came up too quick," as he put it, flicked on the Loran to get his course and coordinates to the destination, and switched on the colorscreen depth-meter, on which he can see individual fish 100 feet down.

Space-age fishing.

We were chatting away a short time later when I noticed land.

"What's that?"

"Hooper's Island," he said. "We're there."

Clearly, Sullivan has it figured. Trouble is, of course, it all comes to big figures.

"I couldn't have built this boat on fishing money," said Sullivan, who owns a profitable printing business in Landover and drives a Cadillac on land.

Nor can everyone afford his fishing trips. To make Dolly Diesel pay for herself, Sullivan figures to run no parties smaller than 12 people, and charges $60 a head. That's $720 a day minimum, a big jump from the $20 to $30 a day most headboats in the area charge per person, and a big jump from the $300 or so charter boats charge to carry six.

But the faint of heart never won the fair lady, as they say. Sullivan is jumping in with both feet. He may be paving a way to keep bay fishing alive for a few more years.

Now, if the world would just stop pouring its filth in the water.