Two people have died after a boat capsized in Moreton Bay off Brisbane.

Emergency services including a rescue helicopter were sent to the scene just after 1pm on Wednesday, with police divers entering the water.

Earlier, Queensland Ambulance Services said one person was flown to Princess Alexandra Hospital in a critical condition.

One of the men was found clinging to the boat and was airlifted to hospital in a critical condition but later died.

Authorities are unable to confirm how many people were on board when the small boat capsized near Green Island between Manly and North Stradbroke Island.

An investigation is underway.

-with AAP