On a bright and sunny Wednesday last week, Don Margolis, 53, slipped into the waters off Aquinnah and swam to Cuttyhunk. Accompanied by his wife and two friends, who traveled alongside him in a small Boston Whaler, Mr. Margolis made the crossing in a little over three hours.

“It’s just what I love to do,” Mr. Margolis said in a phone conversation Tuesday. On July 31, the day of his swim, conditions were “spectacularly perfect,” he said. “No wind, flat [water], warm, Gulf Stream current, 71-72 degrees . . . it was just a delight.”

Mr. Margolis lives in Oakland, Calif., but has summered on the Vineyard for the past 30 years. A regular open water swimmer, he swims year round on a near-daily basis in the waters of San Francisco Bay. Treasure Island and Alcatraz Island are common destinations.

“The water in Martha’s Vineyard is just a bathtub compared to the bay in terms of temperature and conditions,” he said. The current in the bay is five times stronger than that in the sound, and boat traffic is far heavier. The greatest hazard during the Cuttyhunk journey was other boat traffic, Mr. Margolis said, but after navigating past a cluster of fishing boats at Devil’s Bridge, “it was smooth sailing.”

Cuttyhunk, the outermost island in the Elizabeth Islands chain, lies about five miles northwest of Aquinnah. Mr. Margolis said his swim was closer to six miles long because of a stronger current on the Cuttyhunk end. On arrival, he scrambled up on the rocky shore — “I was hoping to reach a sandy beach,” he said — raised his arms in triumph and returned to the Boston Whaler. The return trip to the Vineyard took about 20 minutes.

Mr. Margolis cautioned that such a swim shouldn’t be attempted by the inexperienced.

“I don’t want people to take my experience and be too cavalier about it,” he said. “You really have to plan and train, and have a boater with you.” The boat crew not only offered a physical refuge and navigational help, but a mobile refueling station; his supporters offered water and food, in the form of a liquefied energy substance, to help keep up strength.

“I was inspired by the experience of some of my fellow swimmers who have done far more ambitious things,” Mr. Margolis said. A swim buddy back home has already suggested next year’s trip, which involves a bit more than cross-county travel.

“Next year, I have to swim from Spain to Morocco across the Gibralter strait,” Mr. Margolis said, laughing.