The union representing Canadian Coast Guard employees said the agency plans to close stations in Gimli, Selkirk and Kenora.
Union of Canadian Transportation Employees President Christine Collins said they've been given three possible dates for the closures, ranging from immediately to end of the season.
"It is worrisome that the needs of individual communities have not been reviewed prior to any announcements being made," Collins said.
The union said the closures are because the work no longer falls within the coast guard's mandate.
The Lifesaving Society's Manitoba branch has also condemned the move.
The organization said the leading cause of drowning deaths in Manitoba is boating accidents.
"Navigation services provide the prevention and action to prevent deaths, and search and rescue is there in the case that something might happen, and those are vital assets that we need in Manitoba," said the society's operations manager Kevin Tordiffe.
Without the CCG's presence, it said boating on Lake Winnipeg and Lake of the Woods will be more dangerous.
Wolfe Smythe has been boating on Lake Winnipeg for decades. He said he was shocked to hear the CCG would stop service in the area.
"I was actually appalled," Smythe said.
He said he fears the move could bring more fatalities, from recreational boaters to commercial fishermen.
"Who's making these decisions? They don't seem to be the right ones," he said.
The CCG said there will be no changes to service until the review has been completed.
