Disclosed is a chain tensioner for employment on a bottom of a water body, wherein the chain tensioner is configured for adjustment of the length of a tensioning chain that extends between the chain tensioner and a floating offshore facility, wherein the chain tensioner includes a chain lock for engaging and confining a link of the tensioning chain in the chain passage from aside the tensioning chain, and a chain switch device having a chain guide for engaging the tensioning chain in the chain passage from aside the tensioning chain, wherein the chain guide is moveable between a first position in which the chain lock engages a link of the tensioning chain, and a second position in which the tensioning chain passes the chain lock.