I recently watched the movie and got a couple questions about it, but I'm not sure if it's on-topic.
Should I ask the questions on movie.SE only? Does it depend on the questions?
It is clearly considered on-topic by our community-established guidelines
Guideline #1 is "If it's marketed as SF, it's on-topic." The Life of Pi was marketed as "the next Avatar," and was compared to Hugo. (It's also worth noting that Wikipedia calls the novel fantasy and the film won a Saturn Award for Best Fantasy Film.)
Since the possibility of being fantastical is essential to the theme of the work, the film also fits Guideline #2 (if magic, futuristic science or technology, alternate history, or other sf-nal concept is an important part of the overall plot, it's on-topic).
Even if none of these were true or admissible, Guideline #3 (if the question is specifically about an sf-nal element, even if it's only a minor part of the work, it's on-topic) means that questions specifically about the film's fantastic elements would still be on-topic. (As an aside, I suspect that questions about other aspects of the film, like production or marketing, would be better answered over at movies.se regardless of whether they're also on topic here.)
Fantasy elements and themes in the film
I haven't seen the film, but the whole point of the novel is that a truly fantastical story may or may not be true. It's not overtly fantastical, but it is not something that could just be filed away as non-fantasy fiction.
I believe the phrase is magic realism: "On the surface the story has no clear magical attributes and everything is conveyed in a real setting [...]" but this "highly detailed, realistic setting is invaded by something too strange to believe."
The central premise that a tiger chose to not only not eat but actively assist a young boy while they were isolated and starving is relatively fantastic in itself, but other elements--like the nocturnally carnivorous island clearly mentioned in the wiki summary--are more obvious fantasy fare, in line with the events of an H.G. Wells outing, or a novel of Professor Challenger or Doctor Doolittle.
As for the story's implication that its fantastical elements are merely a hallucination--it doesn't matter. This site has a proud tradition of treating possibly-hallucination-based fantasies as on-topic: alice-in-wonderland, wizard-of-oz, and so forth. By that criterion, buffy isn't on-topic either: "But ultimately the entire series takes place in the mind of a lunatic locked up somewhere in Los Angeles, if that’s what the viewer wants."
I don't think I would consider Life of Pi to be Sci-Fi or Fantasy.
It's heavily implied at the end that the story was mostly made up, and nothing that was made up was very fitting for the 'Fantasy' Genre.
On the other hand, we have a precedent set by this question about Bridge to Terebithia
My understanding (from the Wiki plot and the trailer, NOT watching the movie) is that none of the events in it are fantasy/fantastical. No talking animals, etc... There was a hallucination of mentally connecting with another survivor, but my understanding is that it's implied to be a hallucination even within the fictional story.
Therefore it is no more SFF than "Blue Lagoon" or "Cast away" - it's a fictional adventure story, but not fantasy or science fiction.