Arsonists set light to two buoyancy aides on the canal between Higham Ferrers and Irthlingborough.
The two life-saving rings were set alight and destroyed sometime between Friday, July 25, and Wednesday, July 30.
A police spokesman said: “We would like to speak to anyone who may have witnessed the rings being set alight, or anyone who may have any information about the incident.
“One of the rings has been replaced and the other is in the process of being replaced.”
Witnesses or anyone with information about the arson is asked to call police on 101 or Crimestoppers anonymously on 0800 555111.