SOUTH PORTLAND – John Harvey’s job is to protect people who go in the water. But he gets a chill at the thought of going in the water just for the fun of it.

“The brave ones go up to their knees and do a little dunk or something and run out,” said Harvey, a lifeguard. “I don’t even know why people swim here.”

To the south on the New Jersey shore, swimmers are now frolicking in 80-degree surf. To the north in Canada, on beaches on Prince Edward Island or the northern shore of New Brunswick, swimmers are enjoying water temperatures in the 70s. But in the middle lies the Gulf of Maine and its famously cold water.

“If you get water in the 60s, that’s the tropics here,” said Kevin Sweeney, 57, who swims laps every day at Willard Beach, the coldest beach in Casco Bay.

Several swimmers at Willard Beach quickly blamed the warm Gulf Stream for missing the Maine coast. That’s part of the story. Georges Bank, a Massachusetts-sized underwater plateau, blocks that warm-water current from entering the Gulf of Maine.

But even without a warm current, scientists say, the summer sun is so powerful now that the surface water here should be a lot warmer than it is.

For a full explanation for Maine’s cold water, swimmers should look skyward. At the moon. It is the moon’s gravitational pull that creates tides, and the tides in the Gulf of Maine are unusually strong. Indeed, the tides in the eastern end of the gulf at the Bay of Fundy are the highest in the world, as much as 45 feet in some areas.

Tidal currents mix the surface water with the cold water that lies deep in the ocean, said Andrew Thomas, a professor of biological oceanography at the University of Maine School of Marine Science.

The gulf’s coldest waters are off eastern Maine and the southern tip of Nova Scotia, and in the Bay of Fundy.

Waters off Prince Edward Island are much warmer, Thomas said, because tides don’t create the kind of mixing that occurs on the Gulf of Maine.

Thomas has tested his theory by swimming there. “I can vouch that it is as warm as the beaches in Virginia,” he said.

In the case of York County beaches, the wind plays a role in pushing the surface water offshore, allowing colder water to come to the surface. This “wind-driven upwelling” causes frigid beach days even when the weather is hot, Thomas said.

Mainers shouldn’t complain, though. Sure, New Jersey has warm water beaches. But the Gulf of Maine is among the most ecologically productive coastal areas in the world, Thomas said.