Emergency services were called to the scene between the Kilchattan Bay and Kingarth areas of the Isle of Bute at around 3.45pm today.
Police Scotland said that two people on board have been taken to hospital.
The force said that investigations into the circumstances of the incident are at an early stage.
The Scottish Fire and Rescue service said that it had three fire engines at the scene.
The Scottish Ambulance Service said that one person has been taken to hospital on the island while the other has been airlifted to the Southern General in Glasgow.