The antecedent of it is ambiguous if one suspends the usual mind-reading abilities of language users.
When someone talks, you immediately calculate to yourself: Why are they talking? What are they trying to accomplish by talking? Why did they phrase the sentence the way they did? You know how to read minds, so you answer these questions to yourself automatically. (See Tomasello's Constructing a Language)
In this case, saying "Now I can use it" suggests that there was something the speaker was previously unable to use, but now can. Since they discovered the propeller for the first time, then it is not the thing that they previously thought they were unable to use. So it must be the boat (which is itself a useful thing).