Corrected entry: Later on in the movie after the scientists place Madison in the "fish tank", she stays there for a while until Allen comes to visit her. When he runs up to the tank, and she swims up to the surface, watch all of her fin. Now think, people...would a fish bend his body as if he had knees underneath? It looks as if Madison had knees underneath her fin, since it's bending in that sort of manner.
Corrected entry: Madison spends a significant amount of time in the north. She meets Allen in Cape Cod when he is a boy and rescues him there when he is a man. Also, her undersea city is within a short swimming distance of New York City. With all that exposure to northern climates it makes no sense that she has never seen ice, as evidenced by her comments at the skating rink.
Corrected entry: When Madison turns up at the Statue of Liberty, the audience is assuming she has just come out of the water. Of course she may have waited a few minutes for her fin to dry and turn into legs. But her hair is completely dry. I think with all that hair it would have taken a long time to dry.
Corrected entry: When Allen leaves the wedding reception on his way to Cape Cod, he gets into a taxi. He doesn't have his tuxedo jacket on, nor is he carrying it. In the next scene when he walking on the beach in Cape Cod, he is wearing the jacket.
Corrected entry: Near the end, Tom Hanks, John Candy and Eugene Levy talk their way into the building where Daryl Hannah is being held to rescue her. They wrap her in a sheet and manage to get out of the tank room past the stupid guard. The next scene shows them out on the street getting into Hanks' car. How did they get the sheet-wrapped body past all the other security points on the way out?