“The breeze and the heat and the sunshine, I feel zero risk,” said Ms. Willis, who doesn’t own a boat but has participated in four Trump boat parades in Florida and Georgia. “We can’t go to rallies, but this is a fun way to show him our support.”

President Trump has noticed. Since the pandemic has shelved his signature campaign rallies—a source of energy for the president and his supporters—the splashy displays of approval have helped buoy Mr. Trump’s spirits in recent weeks, aides said. Aides have tried to route the president’s motorcade past boat parades, and are considering having the president participate in one in the coming weeks.

The president has publicly praised the boat parades this summer, including twice during separate coronavirus news briefings, at a roundtable discussion on Venezuela, and while signing legislation to impose sanctions on Chinese officials. In private, Trump broaches the subject of boat parades himself, aides said, asking if they have seen the flotillas of his followers crowding waterways from the Great Lakes to the Gulf of Mexico.