Japanese fishing fleets once supported their nets with floating glass balls made from old sake bottles. Many of those balls are still bobbing around the Pacific a century later. They're big enough that birds, fish, and whales aren't likely to try to eat them.
There are two types of glass bottles in use today: heavy ones intended for re-use, and thin ones intended for single-use. Reusing single-use glass bottles isn't wise, as they break too easily. Recycling single-use glass containers isn't as environmentally-friendly as recycling paper or metal containers. Melting down a recycled bottle saves less than a penny's worth of electricity, compared to making the same amount of fresh glass from sand.
Instead of melting down single-use glass containers, let's just heat them up until the glass gets soft (around 770 C), and then use a puff of compressed air to turn them into glass spheres. Then we'd vacuum-sputter the insides with aluminum to make silver balls, or just squirt some paint inside to make white balls. We'd melt the neck shut, and bake the spheres in an oven to cure the paint and temper the glass. That way, if the sphere broke, it would shatter into sand instead of making jagged shards. We'd then dump the spheres into a river, and let them flow out into the ocean.
We could tie rope to the necks of the bottles before releasing them. Seaweed and shellfish would grow on the rope, flourishing even after the sphere drifted far away from land. Clusters of balls would form floating reefs, sheltering and nourishing fish. If we plugged the necks with something rougher than glass (concrete?), coral might even grow.