The boat overturned on Sunday evening between the towns of Sesto Calende and Arona, Italian media say.
Five people were rescued and taken to hospital.
Unconfirmed reports in Italian media say the victims are Italian, Israeli and Russian.
One of the them, a Russian woman, is said to have been the partner of the boat's skipper.
Attilio Fontana, president of the Lombardy region, said the "very serious incident" was due to bad weather.
The boat was 16m (52ft) long and had been hired by tourists, he said on Facebook.