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troperville

When Madison must go back to the ocean, she tells Allen that he can go with her, but he will never be able to return to land. It is never explained why, perhaps it may have to do with the sea hag subplot. Still, he was reluctant at first but accepts readily after saving her from government frogmen. It's somewhat a Bittersweet Ending; the bitter part is Allen will never see his brother, friends, or even Kornbluth again. The sweet part is being with Madison always. So, the question is, why couldn't he return?

It's explained in the sequel, as Madison reveals that she was just kidding and that it's actually perfectly okay for them to go back to land, so it's possible it was a test question, how much would a human be willing to give up for his aquatic lover? Once Madison realized Allen was really going to stay with her, it was safe to tell him the truth.

How could Allen breathe underwater? Was it the kiss?

Seems like it. YMMV, but Pirates of the Caribbean: On Stranger Tides seems to contain several Shout Outs to Splash, such as mermaids turning human when dry, and back to mermaid when wet. It is explained in the movie that a mermaid's kiss prevents drowning. It is implied to be the same in Splash.

Spider-Man

Headscratchers/Film

Splice