Coast guards are searching for the 26 people on board after the vessel sent out a distress signal.
Japan’s coast guard are searching for a tourist boat missing off the northern island of Hokkaido, reports say.
Authorities lost contact with the vessel, which had 26 people on board, after it reported that it was taking on water at 13:15 local time (04:15 GMT).
The Kazu 1 was tipping at a 30-degree angle and starting to sink, the crew reportedly said.
The boat is believed to have been on a three-hour sightseeing voyage around the Shiretoko Peninsula.
The Japanese coast guard has dispatched five patrol boats and two aircraft in a search of the area.
The 24 passengers are said to include two children and there were also two crew members aboard the boat.
Waves in the area had been high and local fishing boats had apparently decided to return to port by mid-morning.
The crew reportedly said that all those on board were wearing life jackets.
But temperatures in the area can dip as low as zero degrees Celsius (32 degrees Fahrenheit) as night falls.