Piper - The background is made as so damned real. The story is simple but meaningful.
Flaws are including not many species of fish living in salty water can survive in fresh water. And how can the pipes of Marine Life Institute connect to sea outside directly? But without these, the story cannot go on.
The surprise brought is weaker than Toy Story 3 or Inside Out. I'm really working on how the story is in Cars 3 and Toy Story 4.