This is a movie intended to run in the background, and was never intended to be watched exclusively.
Turn it on for young kids-- it makes a pleasant moving background with ocean sounds to help bring ambience while kids play or during a birthday party.
The characters of Finding Nemo and Finding Dory appear on screen periodically and interact with each other and with the camera, making it interesting for a child whose imagination wanders from activities to the screen for a minute or two.
Anyone who gives this a low rating doesn't understand what it's intended for. There are other types of movies similar to this, such as "Cat warming itself in front of a hearth" movie, which is also three-hours long. Obviously it's not intended to be watched for three hours.