I guess they used 5 dogs for the movie, so maybe the dog in the water is a different dog. I don't know.
As for a dog's purpose, well, it seems like perhaps enough info to provide for a new thread. I wasn't intending to watch the movie. I figure it is so much blither to try and reduce the most tender-hearted to blithering. But do dog's have a purpose? Should they?
I was thinking about this this morning. So many people see them as furry kids. My dogs are so excited for me to come home every single day, rain or shine, no kid does that. No parent would expect a child to be so absorbed in themselves that they would be waiting for the highpoint of their day, momma's home! But dogs are just like that. It doesn't matter if you have 1 or 20, they are thrilled when you come home, thrilled when you pay them attention, thrilled to go with you, thrilled to train, thrilled to play, thrilled to eat, thrilled to climb up in your bed and give you wet doggy kisses.
It may not be a dog's purpose in life to make a human's life worth living, but I think many people have stumbled through rough patches because of their dogs. I think they are so suitable to accompany humans through a portion of their lives, that we think of their purpose as companion.
I think we set ourselves up for disappointment if we expect our dogs to fill a hole in our lives, but many dogs do fill many holes in our lives.
I'd say a dog's purpose is to attach itself to other critters. In doing so, it is provided for and it provides companionship. If the other critters are all critters/dogs, then it will work together with other critters to make finding food easier. Attachments will provide in many cases for reproduction of the species, better health, etc. The attachment, the instinct to attach to others is what makes them so suitable to us, for many things.