What does "let it go" mean?

I feel like there's a subtle difference in meaning between "contentious" and "controversial". Could you explain this more?

Good question! The answer is that in this context, "contentious" and "controversial" mean basically the same thing. In regards to issues, you can use the two words interchangeably. Ex: Abortion is a very controversial topic in America. Ex: I don't like speaking on contentious topics. When used to describe people, the meaning is different. A "contentious" person is someone who is argumentative. Meanwhile, a "controversial" person is someone that arouses controversy, someone that OTHERS argue about. Ex: She is known for her irritating, contentious personality. Ex: Winston Churchill is a bit of a controversial person.

Does "interpret" mean same as "understand?"

It's very similar to "understand!" "Interpret" means to understand something in one's own way, or the way they understand things. One decide what the meaning of something is. Ex: I interpret this abstract painting as a butterfly. How would you interpret it? Ex: The text can be interpreted in many different ways, depending on who's reading it.

Instead of “I got no words”, can I say “I have no words”?

Yes, "I have no words" can be used instead of "I got no words". "I have no words" is correct grammar, while "I got no words" is a slang phrase.

Why did he use “did” in this sentence? Is it grammatically needed?

"Did" in this context is used for emphasis to reaffirm a statement. It puts the sentence in a question-like form with "did" to give it emphasis. It is necessary in this context to use "did" unless the sentence is reworded. Ex: Did I see her? Yes I did. Ex: Not only did I watch the performance but I filmed it too!

Is there any other words I can use instead of "steer"?

Yes, there are other words you can use in place of steer in this context, such as "maneuver", "guide", "navigate" or "direct". You can also say "row" since he is using a paddle to row the boat in this context. The verb "steer" is usually used when there is a wheel involved with the movement or direction of a boat or vehicle.