“But he that hath the steerage of my course direct my sail.”
The metaphor here is that Romeo is a boat. Alone in a large ocean, alone and completely alone. The boat has no control over the waters of fate but “he that hath the steerage” can navigate the waters of fate and decide on the location. It is always a planned before however. You can’t just set off without preparing. This is why the navigator will know the destination before the journey. It could be planned months ahead, maybe years. But Romeo doesn’t know this course. He is just a vessel in his mind. He is making God steer him to wherever planned. The same metaphor is used in both the book and the movie. It has to have a significant meaning in the story then.
In the movie the scene that the metaphor appears is where Romeo gives up to his decided fate and to let God take over. He believes it is futile for him to resist fate and so he takes an unknown drug and he is left in the worlds control. He makes the metaphor at that moment because he believes that something major is going to happen at the party. He cant stop what will happen because it was told aeons ago. That how fate works.

Kahu,
Your observations about the metaphor are good. You clearly apprehend Shakespeare’s intention.
Where this paragraph needs work is in its structure. In order for your ideas to be conveyed clearly, it’s a good idea to use a strict structure when you write analytically.
We recommend that you start with the statement, example, explanation structure as this matches the readers’ expectations of this kind of writing.
Can you please highlight all the sentences in your paragraph above that you consider are statements, and in another colour highlight all that you think are explanations.
Once you’ve done that, we can work on re-organising things.
CW