Thomas (TJ) Mello, 49, of Tisbury is a familiar Vineyard face. For 29 years he worked as a clerk for the Steamship Authority. He would check people in as they lined up to board the boat in Vineyard Haven, always offering a smile.

He retired last May, turning his attention to his love for being on the water. “I think I’ve checked everyone in on the ferry at least once,” Mr. Mello said in an interview.

This summer he is a full-time captain aboard the county sheriff department’s courtesy patrol boat, a 23-foot C-Hawk with a 225-horsepower outboard.

“I love doing this,” Mr. Mello said. He has been working for Sheriff Michael A. McCormack now for three years.

The boat operates out of Edgartown.

Throughout the summer, Mr. Mello is out on patrol, urging boaters to wear their lifejackets, helping disabled boats when needed and giving directions. Mr. Mello is a licensed captain who has spent a lifetime on the water, going back to the days when he puttered around Lake Tashmoo.

His mate this summer is Ray Perry, 18, a recent regional high school graduate.

Last Saturday, he assisted in the rescue of a captain and two passengers on a 39-foot Grand Banks Eastbay SX boat that sank quickly between two government buoys in the outer harbor, after it hit a charted rock just off the channel.

In late July he rescued two kayakers three miles north of State Beach. A 56-year-old father and his 14-year-old daughter had gone out in two small kayaks not suited for offshore paddling. “There was a strong southwest wind. They couldn’t paddle back to the beach,” he said. A fetching sea flipped and swamped the father’s kayak; he was in the water clinging to both kayaks. “He was so tired he could barely lift his hands out of the water,” Mr. Mello said.

Mr. Mello sees his job as assisting harbor master Charlie Blair’s very able crew. His work is more about public safety than enforcement, he said.

He said boaters in their first year on the water are most inclined to get into trouble. “It isn’t just first-time boaters, it is first-season boaters that get into trouble,” he said. “They have a hard time doing the simplist things, connecting their boat to a mooring, or launching,” he added.

“Know where you are,” Mr. Mello said, calling it the first rule of boating.

He thinks he knows where the high-speed powerboat hit a rock last Saturday. He said it is probably Mill Rock, one of a number of charted and named rocks west of the channel between the two government buoys R2 and R4.

There is also Allen Rock, Hatsett Rock and Monohanset Rock.

Mill Rock is notorious; Mr. Mello said the late harbor master John Edwards used to mount a tree on it. Because it was a pine tree, the rock earned the name Christmas Tree Rock.

Mr. Mello recommends the use of charts and GPS for boaters to carefully plot their course.

And one more thing not related to technology.

“Mariners still look out for each other,” he said. “When that boat sank last Saturday, there were a lot of boats on the scene offering assistance. If you are a boat operator and you see someone in trouble, you’ll go over and ask them if they are okay.”