Mr. Clinton was seen kneeling to pet a dog when he arrived at Walker's Point, the summer home of George H. W. and Barbara Bush on Maine's rocky coast.
Later, the former presidents waved to onlookers early Monday evening as they boated up and down the river that separates Kennebunk and Kennebunkport before roaring into the ocean for a short ride in Bush's three-engine boat, the Fidelity III.
The 1992 election rivals became friends when they led fund-raising as part of the relief effort for victims of the Asian tsunami. Bush extended the invitation after learning that Clinton would be in Maine for a book signing.
Earlier, customers at a Borders bookstore near the Maine Mall cheered when Clinton arrived wearing slacks and an orange Polo shirt.
About 70 people already were in line when the store opened and the line snaked around three sides of the building by noon. The first 500 were permitted to meet Mr. Clinton and have him sign their copies of his memoir, "My Life."
"I feel great. I've been working hard," said Mr. Clinton, in his brief remarks to reporters, saying his tsunami mission included visits to a dozen countries in 15 days.
Those who came away with their signed copies of the Clinton memoir and the memory of a handshake with the ex-president said it was worth the wait.
That was doubly true for Betty Pomroy of Portland, who also got the former president to sign a photo of himself and her son Colin, a Verizon employee, taken at the White House.
"It's like a dream come true," she said.
The Kennebunkport portion of the trip was private, but Mr. Clinton noted it wasn't his first visit to the Bush seaside estate.
He recalled being there in 1983 as part of a group of governors that visited Mr. Bush when he was vice president in the Ronald Reagan administration.