Tool use is not very common in non-human animals, snd in many cases, it is unclear whether the animals are using tools as the result of inherited motor patterns or are actually using learning too use through observation or reasoning.
Chimpanzees, bonobos, and orangutans have been seen using tools as a result of their novel intelligence. They clearly learn tool use through observation.
Now, I think that is the latter type of tool use that we’re seeing here. This dog probably watched children use that raft and then decided to use it to fetch without getting too wet.
This is actually a more sophisitcated and unusual behavior than you might suspect.