WordNet (r) 3.0 (2006):

lifeguard
    n 1: an attendant employed at a beach or pool to protect
         swimmers from accidents [syn: lifeguard, lifesaver]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "lifeguard":
   air warden, air-sea rescue, attendant, caretaker, castellan,
   coast guard, conservator, curator, custodian, forest ranger,
   game warden, gamekeeper, governor, guardian, guardian angel,
   janitor, keeper, lifeboat, lifesaver, next friend, prochein ami,
   ranger, rescuer, savior, shepherd, steward, warden, warder