A Canadian scuba diver has been arrested by U.S. authorities after swimming across the St. Clair River with about 3.6 kilograms of marijuana stuffed in a waterproofed piece of pipe.

A U.S. Customs and Border Protection spokesman said that a boater spotted a swimmer in the river around 1 a.m. on Monday. The boater contacted local police, saying they had spotted someone who appeared to be swimming toward the U.S. side of the river.

U.S. border patrol agents apprehended the Canadian, dressed in a full scuba suit, when he got out of the river at Marine City, Mich. (U.S. Customs and Border Protection)

The police contacted border patrol officials, who then contacted a command centre that operates cameras, Kris Grogan said.

"Our camera operators were able to locate the individual with night-vision cameras," Grogan said.

They sent the location to agents, who were able to meet the man as he climbed up the seawall near Marine City, Mich.

At the time, he was dressed in a full scuba suit, Grogan said, and also had a section of an airtight, watertight PVC pipe which had about 3.6 kg of marijuana stuffed inside.

"In that part of the river, it's not a short swim," said Grogan, who estimated the distance to be about a kilometre. "He probably drifted a ways too, it's a very strong current through there as well."

The case has been turned over to the St. Clair County sheriff's drug task force, with charges expected to be announced later this week.