WordNet (r) 3.0 (2006):

life preserver
    n 1: rescue equipment consisting of a buoyant belt or jacket to
         keep a person from drowning [syn: life preserver,
         preserver, flotation device]

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "life preserver":
   Carling float, Mae West, aegis, arm guard, backstop, balsa,
   balsa raft, bob, boom, breeches buoy, buffer, bulwark, bumper,
   buoy, contraceptive, copyright, cork, cork jacket, crash helmet,
   cushion, dashboard, dodger, ejection capsule, ejection seat,
   ejector seat, face mask, fender, finger guard, float, foot guard,
   fuse, goggles, governor, guard, guardrail, hand guard, handrail,
   hard hat, helmet, insulation, interlock, knee guard, knuckle guard,
   laminated glass, life belt, life buoy, life jacket, life net,
   life raft, life ring, life vest, lifeboat, lifeline,
   lightning conductor, lightning rod, mask, mudguard, nose guard,
   pad, padding, palladium, parachute, patent, pilot, pontoon,
   preventive, prophylactic, protective clothing, protective umbrella,
   raft, rubber dinghy, safeguard, safety, safety belt, safety glass,
   safety plug, safety rail, safety shoes, safety switch,
   safety valve, screen, seat belt, shield, shin guard, sun helmet,
   surfboard, umbrella, water wings, windscreen, windshield