Some of the lanterns were for pets lost in the past year.
These are candies specifically for Obon. These had azuki bean filling. I saw them for sale everywhere in the weeks leading up to Obon. I thought they were candles or the kind of soap that people put in their bathroom that you're not sure if you're supposed to use or not. One of the first things we were told when we moved to Japan was that we should plan to travel during Obon. Many people use that time to travel to be with their families and the roads are packed.
At first the lanterns started floating up the river and away from the ocean. No one was too concerned as they go out toward the ocean eventually. Our guide told us that there was a team of people waiting around the corner to grab up the lanterns so as not to pollute the ocean.