The film is worth seeing.

I'll explain why "seeing" is bad. "Rewarding" acts as an adjective to modify "film". However, films do not see, so "seeing" cannot modify "film". The only other possibility is for "seeing" to be an adverb modifying the adjective "rewarding". But that feels wrong too because "seeing" doesn't describe well how or why the film is rewarding. Thus it is difficult but perhaps not impossible to defend the sentence.

The film is rewarding to see/worth seeing

Let me explain one more reason the sentence sounds bad. Whenever you use two present participles (like "rewarding" and "seeing") consecutively, the listener or reader will interpret them as modifying the same word. Since "rewarding" modifies "film", the reader will assume that "seeing" also modifies "film", leading to a nonsensical interpretation. Always use consecutive present participles in the same way. Here is another example. Consider this terrible sentence: "The boy saw a cat crossing the road riding a bicycle." Since "crossing the road" modifies "cat", a listener will automatically assume the same to be true of "riding a bicycle", leading to the nonsensical conclusion that the cat was riding a bicycle.