Going by the dictionary, it depends on how you interpret it.
Depending on how you interpret it, the OED arguably lists it.
informal A thing that saves one from serious difficulty.
‘a microwave could be a lifesaver this Christmas’
Australian/NZ A lifeguard working on a beach.
The first definition is very vague, intended to describe figurative lifesavers, but I think you can argue that it doesn't necessarily exclude literal lifesavers (e.g. a "swim ring").
In my opinion, it's counterintuitive to allow the figurative usage but not the literal usage, especially when the figurative meaning expands on the literal meaning.
If you can consider a microwave a lifesaver (for improving your life), then you should also allow for a "swim ring" to be considered a lifesaver (for saving your actual life, which can still be considered an improvement to your life).
The same argument can be made for Dictionary.com:
- a person who rescues another from danger of death, especially from drowning.
- a person or thing that saves a person, as from a difficult situation or critical moment
- (Chiefly British) a lifeguard.
Definition 2 is similar to the vague definition found in the OED.
And Merriam-Webster lists a similarly vague description that could fit the bill:
- one trained to save lives of drowning persons
- one that is at once timely and effective in time of distress or need
It does seem to be in active use, regardless of dictionary definitions.
If you forget about the dictionary, "lifesaver" seems well established (when referring to the "swim ring") when I look it up online.
- As mentioned by others, Life Savers are also a piece of candy. However, it's important to note that the name of the piece of candy is based on its resemblance to the "swim ring". Wikipedia: "The candy's name is derived from its similarity to the shape of life preservers used for saving people who have fallen from boats.". This indirectly proves that the "swim rings" must have been colloquially referred to as "lifesavers" (at least at the time when Life Savers were named).
- Looking at the Google image results for "lifesaver", it consistently shows the "swim ring" as a result, with a few images referring to the candy.
- Anecdotally, I'm not a native English speaker but I immediately understood what you were referring to even before I saw the picture.
- Wikipedia lists "lifesaver" as a synonym for "lifebuoy".
In my opinion, "lifesaver" is correct to use when referring to a "swim ring". Others may disagree due to a lack of dictionary definition.
Are there better synonyms available?
Forgoing the subjective (and neverending) discussion about whether a dictionary definition is required to consider "lifesaver" correct, let me offer some alternatives. Note that while synonyms exist, I haven't yet come across one that I would personally label as better than "lifesaver".
Wikipedia primarily lists it as a lifebuoy. It also offers some synonyms:
lifebuoy, ring buoy, lifering, lifesaver, life donut, life preserver or lifebelt, also known as a "kisby ring" or "perry buoy"
Here's another Wikipedia article, referring to it as a personal flotation device.
However, this doesn't just refer to "swim rings", but every type of device that help you stay afloat (e.g. life vests, torpedo buoys)
According to the OED, "lifebuoy" is equally vague, referring to any device that help people stay afloat:
A buoyant support such as a lifebelt for keeping a person afloat in water.
The only ones that come close to exlusively referring to a "swim ring" are lifering and life donut, since both "ring" and "donut" inherently suggest a toroidal shape.