Art burglars who stole a work by 17th Century master Anthony Van Dyck may have used a boat to escape after the heist, police have said.
A Soldier On Horseback by the Flemish artist, a leading court painter under King Charles I, dates from around 1616.
The work was among three stolen from the University of Oxford's Christ Church Picture Gallery on Saturday.
Thames Valley Police said a boat could have been used "to access and leave the site".
The thieves also made off with A Boy Drinking (c. 1580) by Annibale Carracci, and A Rocky Coast, With Soldiers Studying a Plan (late 1640s) by Salvator Rosa.
Det Insp James Mather said: "This is just a hypothesis at this time, but we would like to hear from anyone who has had their boat stolen recently or has noticed any unusual activity around where their boats are docked.
"We are initially asking people with boats based on the River Cherwell or the River Thames near Oxford to get in touch if you have noticed any unusual activity.
"If you saw anything unusual on these rivers on Saturday night, we would also ask you to get in touch.
"Also, please make a report if you find any abandoned boats which have appeared since the early hours of Sunday morning."
A Christ Church College spokesman said staff had initially alerted police to the theft of the "important cultural artefacts", and the gallery would be closed until further notice.