Television footage showed the upturned hull of a small fishing boat in the Seto Inland Sea off Hiroshima after what the coastguard said was a collision at around 8:00 am with the Osumi, a naval transport ship.
All four crew from the fishing boat were pulled from the water.
A coastguard spokesman said two of them were conscious, but the other two -- including the captain -- were in cardio-respiratory arrest. The term is usually used by first responders of people who have died but have not yet been certified by a doctor.
The Seto Inland Sea is a relatively busy waterway that serves a number of major ports, as well as supporting a vibrant fishery.
