A WATER Police officer dived into Port Phillip Bay to save a woman and then promptly arrested her for allegedly stealing a boat from the St Kilda Marina.

The drama began at around 11am when a fisheries officer saw a boat run aground off Williamstown.

He recognised the vessel as belonging to a fisherman and made contact with the owner.

The boat had allegedly been stolen from the St Kilda Marina on last night.

Water Police called to the scene saw a woman struggling in the water about 500m from shore, and senior Constable Andrew Folley dived in to save her.

The 27-year-old Deer Park woman was arrested after it was determined she did not need medical assistance.

Police say another officer boarded the stolen vessel and arrested a man.

The 38-year-old North Melbourne man and the woman are expected to be remanded in custody to appear in the Melbourne Magistrate's Court on Monday.

The stolen vessel suffered extensive damage and had to be towed to Williamstown.

