WordNet (r) 3.0 (2006):

lifesaver
    n 1: an attendant employed at a beach or pool to protect
         swimmers from accidents [syn: lifeguard, lifesaver]
    2: a life preserver in the form of a ring of buoyant material
       [syn: life buoy, lifesaver, life belt, life ring]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "lifesaver":
   Audubon Society, National Wildlife Service, Sierra Club,
   air warden, air-sea rescue, attendant, caretaker, castellan,
   coast guard, conservationist, conservator, curator, custodian,
   deliverer, forest ranger, game warden, gamekeeper, governor,
   guardian, guardian angel, janitor, keeper, lifeboat, lifeguard,
   next friend, preservationist, preserver, prochein ami, ranger,
   rescuer, safekeeper, saver, savior, shepherd, steward, taxidermist,
   warden, warder