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.
Woman charged over theft of campaigner’s car ‘must stay out of Belfast’
Bluetongue restrictions in Northern Ireland will be relaxed from Friday
Conviction led to safeguarding review at Presbyterian Church
Coroner vows to keep Noah Donohoe inquest ‘on track’ for January start date
Appeal to European Court being considered after ruling over Troubles disclosures