Vandals damaged an inflatable boat belonging to the borough’s Sea Cadets which was being stored at Lord Derby’s estate in Prescot.
The damage was discovered last Sunday (March 20) at around 2pm.
Security staff at the stately home, the grounds of which includes Knowsley Safari Park, searched the area but the culprits had escaped.
Detectives have being trawling through CCTV footage and have released stills of two people they want to speak to.
Insp Kirsty Jennett said: “This was a mindless piece of vandalism which has caused disruption to the training of the Sea Cadets, a charity that provides personal development and confidence to many local young people.
“We are keen to speak to the youths pictured at the earliest opportunity. Advice and reassurance has been given to security on the estate, and we have supported them with extra attention by local officers.”
Anyone with information is asked to speak to neighbourhood officers on 0151 777 6288, call 101 or Crimestoppers anonymously on 0800 555 111.