Once upon a time, before the invention of plastic, all containers and many other items were made out of glass.
Just like its modern, petroleum-based replacement, much glass ended up in the ocean. The difference is rather than float, glass sinks and is pummelled by tides for years, sometimes decades, until the glass becomes smooth and frosted.
Sea glass washes up on the beaches of Victoria and is used in art by a variety of hobbyists and professional artists.