Foyle Search and Rescue said one of them could not be recovered, resulting in the local charity having to replace it.
They said lifebelts have been used on 35 occasions to save the lives of members of the public.
In a social media post, they asked anyone who sees people damaging the items to contact the PSNI.
Education Authority boss hits out at criticisms of public services
Man in 90s dies following Co Down road crash
Police to increase patrols after second incident of shots being fired in Belfast
Woman charged over theft of campaigner’s car ‘must stay out of Belfast’
Bluetongue restrictions in Northern Ireland will be relaxed from Friday