The DeHavilland DHC-3 Otter turboprop crashed under unknown circumstances above Ella Lake near Ketchikan, Federal Aviation Administration spokesman Ian Gregor said in an email to The Associated Press.
The conditions of those on board weren't immediately known, Alaska State Troopers said.
Coast Guard Petty Officer Lauren Steenson said the agency received a report around 2:15 p.m. that the plane was overdue. Troopers said an emergency locator transmitter activated in the Misty Fjords National Monument, and a helicopter pilot spotted the downed aircraft above Ella Lake, about 800 miles southeast of Anchorage.
Promech Air, an airline based in Ketchikan, operated the shore excursion sold through Holland America Line, the cruise ship company said in a statement. The eight passengers are guests on the Westerdam, which is on a seven-day cruise that departed Seattle on Saturday.
"We are incredibly distressed by this situation, and our thoughts and prayers are with those onboard the plane and their families," the statement said. "Holland America Line is extending its full support to travelling companions of the guests involved."
The Ketchikan Daily News reports the Westerdam had been scheduled to leave the city about 20 miles from the crash site at 1 p.m., but it remained in port Thursday evening.
Promech Air didn't have an immediate comment beyond confirming that the downed plane belonged to the company.
The airline's website advertises tours of the 2-million-acre Misty Fjord National Monument in its float planes.
"Towering granite cliffs, 1,000-foot waterfalls, lush and remote valleys and serene crystalline lakes make up this incredible landscape," it says.
Associated Press writer Kathy McCarthy in Seattle contributed to this report.