Captain Arthur A. Small Jr. loves the Potomac River and maybe has spent more time on it in the past 21 years than anyone else.

His eyes never leave the river as he stands at the wheel of the excursion ship "Diplomat" and says "It's cleaning up some. People are getting some fish out of it. And there is some swimming a little way down."

From the end of March to the end of November, Small pulls away from the Maine Avenue pier twice a day (and usually a third time in the evening), gives two toots of his whistle, and heads toward Mt. Vernon.

"I put in about 18 hours a day, seven days a week," he says. "About 700 miles up and down the river."

Small may be the perfect picture of the old Yankee sea captain: a tough, 71-year-old bantamweight with a shock of white hair and a trimmed white beard accenting a tanned, leathery face. He wears a peaked nautical cap with scrambled eggs on the bill.

He squints often, and his eyes are marked with the crow's feet of a man who has peered through every element nature could toss at him in his 61 years on the water. You get the feeling that if he went looking for Moby Dick you would sign on with him.

He's traveled the seven seas, and the "Diplomat," the ship he has grown to love, has a seagoing legend equal to his. His carefully lettered orders on the bulletin board call for a full scrubdown for his lady friend, and a cleaning out of the scuppers, before their trip down river each morning.

The river has been quiet for the two as they age gracefully.

It was a bright, clear morning when he pulled into the channel for the trip down-river. His lunch for the day was to his right, on his chart table; a chopped egg salad sandwich and two containers of milk were backed up with two small blue plastic bottles of liquid antacid.

He kept alert for the power boats and sail boats that skimmed about. His responsibility for that day, as most days, was the standard group of tourists and a group of grade-schoolers who clamped their hands over their ears and smiled widely at each other when he tooted his whistle.

"I have an inland-waters pilot's license," he said. "I can travel any inland water in the United States: the Great Lakes, anywhere, including the Reflecting Pool If I can get up there."

Captain Small was born in 1908 in a little house next to a lighthouse in Boston Harbor, where his father was the keeper. His early schooling took place across the kitchen table from his mother.

"My brother and I learned a lot from her," Small said. "When it was time to go to school on the mainland, I was jumped ahead three grades, and after one year I was jumped ahead three more grades. It seemed to be enough schooling for me."

When he was 10 years old he had begun going to sea during the summers, on a four-master trading schooner. By the age of 13 - he was living in New Bedford then - he was "pretty restless." Then, trouble.

"I don't remember what I did but she laid the strap on me one day," he said, and he moved to point to his right buttock. "I still have the scar. I ran away that night."

The next morning Small shipped out on a four-master schooner headed for trade along the west coast of Africa. "Sailed right out from under my old man's nose," he said, laughing.

Small later fished out of Gloucester, then switched to steamboats and plied the East Coast trade routes, later working on private yachts as a cook and eventually as captain. He sailed out of Booth Bay Harbor, Maine as skipper of a four-master trading ship, served as a lieutenant in the Pacific during the war, then returned to private yachts. While aboard one in Washington he met the Wilson Line owners and was hired as a captain.

By this time, the captain is tying up at the foot of Mount Vernon. Though he does it hundreds of times each year, he has not been up to see the house in years. "The climb up the hill is too long," he says.

Te talks about his favorite ship instead. You know when Small says "she" it's the "Diplomat" he's talking about and not his wife, two daughters or two great grand-daughters. He adds: "I don't know where my wife is. I think New Bedford."

He knows where the ship has been, though. "Around the world twice and through the Panama Canal a few times. She originally ran on steam; now it's diesel."

"She" was lunched in 1930 on the Harlem River in New York City, then turned over in 1942 to the Navy, which converted her for coastal duty by placing three small guns on her bow and depth charges on her stern and changing her identity from "Stevana" to YP255.

Her salad days came to an end in 1945 when she was purchased by a fishing company and given the unladylike name of "Ace". Her once-proud decks promptly became covered with dead fish.

Later she was bought by the Circle Line in New York and dubbed "Sight Seeing Two." From 1959 to 1975 she was the "Diplomat" of the Potomac. Then, after a stint as the "Bostonian," carrying passengers from Boston to Provincetown, she was brought again to Washington by the new owners of the Wilson Line.

She was given her old name, an extensive overhaul, and a fresh coat of paint. On March 31, she and Captain Small, who gave two toots, took off down the river again.

"I helped save a couple of boys after their sailboat turned over," he said, minding his helm again. "They were clinging to a channel marker. It was a cold day. We stopped, radioed the police, and they were there in minutes.

"Rosalynn Carter had a luncheon on board. Secret Service everywhere. Navy divers came to inspect the hull. We had the best caterers and the "Diplomat" looked beautiful. The whole thing cost $45,000 and when we tied up she came up here to the pilot house to thank me personally."

He reaches to show a framed engraved White House birthday card. "Got this on my 71st, signed by the president and first lady."

Small seemed apologetic for the short hour-and-a-half trip down the river, tame stuff compared to his younger days at sea. "The only place I've missed in my travels was not worth looking at or was immoral,' he says.

At night, Small checks every detail of the "Diplomat," making sure she is secure. He walks the seven miles from his home in Alexandria every morning, weather permitting, carrying his walking stick with a jaunty air.

But on the way home he accepts a ride to his favorite bar and restaurant, Mason's in Alexandria, where there is always a seat set aside for him with a plaque on the bar saying "Reserved for Captain A. A. Small Jr."

He likes Beefeaters after the long, 18-hour day, and after having a couple he takes the short walk home to his apartment, maybe to cook himself a gourment meal, one of his hobbies, or just rest and read about naval history. CAPTION: Picture, Cpt. Arthur A. Small Jr.