The man is mute
- It's not just an artistic choice that keeps the characters from talking; the man is actually mute. He has some medical condition that allows him to make noise but prevents him from pronouncing words. (The turtle doesn't speak because she's a turtle and wouldn't know his language anyway.) The man always felt isolated from other people because of his muteness, but now he's on an island and there's no need for words. This is part of the reason why he eventually decides to just live on the island instead of going back to civilization.
The rest of the world is dead or dying
- While the man was at sea, the rest of the world came down with bubonic plague or something. This is why the Red Turtle prevents the man from leaving the island.
It's set in the same universe as Big Fish & Begonia
- Begonia is about a girl from an undersea kingdom who comes to the surface in the form of a red dolphin. It's not too far fetched to say some of them appear as turtles.
