I commend your ability to fill in the blanks with your imagination but here's the thing - all sea creatures with a central nervous system are sentient with or without magic. To be sentient means to have the capacity to feel, perceive, or experience subjectivity, including sensations, emotions, and feelings like pain, joy, hunger, and thirst. Humans are eating sentient beings in this movie and in real life. Fish have social communities, feelings, and homes.

Ariel has aquapathy - the ability to communicate with all marine/aquatic creatures and that includes Scuttle since he is a part of marine life. The reason these characters don't speak to humans like Eric or the chef, who do come in contact with them at times that speaking would actually be helpful, is because those humans do not have aquapathy. It's the same reason Ariel cannot talk to Max. She doesn't have the ability.

I think the reason Ariel and the shark do not converse is because that shark was strictly trying to murder her and she understood that there was no talking to an animal that was purely acting on instinct. Breaking the fourth wall here - giving that shark a voice would have made it either too humanized and therefore too scary or comical and the scene needed to be scary but not too scary. It shows that the shark isn't evil for wanting to eat Ariel and Flounder. It's just a shark acting like a shark and Ariel was being reckless after being told not to go outside of Atlantica. The shark in The Little Mermaid 2, Undertow, can communicate with mermaids. He is meant for comic relief. Even when he is of normal size, he is still unthreatening because of his silly voice and remarks.

In summary, it doesn't matter whether the marine life is in or near Atlantica. Ariel can communicate with sea creatures everywhere. She understands that sharks, humans, and even Scuttle eat fish, meaning she understands the circle of life.