The 6-metre Kiwi-Kraft aluminium boat, parked in the back yard of a Worcester St property, was doused in petrol and set alight early on Tuesday. It was valued at about $30,000.
Police found a grey 20-litre plastic container, stamped with the brand name Fuchs, at the scene.
Anyone who recognised the container or knew where it was from was asked to contact Detective Kevin Kirkman at Oamaru police on (03) 4331418.
Police were also interested in talking to a man who was seen walking along Worcester St between about 2pm and 3pm on Monday.
He was caucasian, of slim build and wearing a brown leather stockman-style hat. That man, or anyone knowing his identity, was also asked to contact Det Kirkman.