innertube
A flotation device used as a recreational swimming aid, especially by young children and older persons who cannot swim. So called because it resembles (and in earlier times, usually was) the inner tube of a car tire.
On Danbooru, often constructed of transparent material, ostensibly for better visibility of the wearer.
A lifebuoy is visually similar, but its use and usual material composition are different.
See also
This tag implicates swim_ring (learn more).
