PORTSMOUTH — A 22-year-old Fall River man died yesterday after falling from his small boat off Portsmouth, environmental police said.

Just before 5 p.m., environmental police said they heard radio chatter that a man was in the water off Dyer Island.

According to police, he was alive when he fell into the water, but was quickly struck by something — police would not say if it was the propeller, the boat, or another object — and was killed.

There were four people on the boat. Environmental police did not release details on the man’s identity, the size of the boat, or what struck the man. They also would not say if there was alcohol involved.