The deliberate fire was started on a vessel on the canal towpath in Oxford.
A man in his 70s and a woman in her 60s were the victims of an arson attack on their vessel, moored on Canal Street in Oxford, on Saturday morning.
A passerby spied the fire and alerted the elderly couple and, with the help of others, they extinguished the flames.
At around 2:20am, the fire service called the police to a fire at a shed 1.5miles along the same towpath.
Police believe that the boat was targeted between 12:30am and 2am.
No one was hurt in the attacks.
A shed near Godstow Road bridge in Wolvercote was also burnt down.
Read about how to tackle on-board fires, including the five types of fire extinguisher that should be carried in your vessel.