A lifesaver.
An attendant, usually an expert swimmer, employed to save swimmers in trouble or near drowning at a body of water.
A sturdy metal bracket fixed in front of each of the leading wheels of a train to deflect small objects away from the wheels to prevent derailment.
A bodyguard or unit of bodyguards, a guard of someone's (especially a king's) life or person.
Someone or something that saves lives.
Someone or something that is very useful or helpful.