Look to the bright side! That's one of most interesting parts in English study.
I believe every single word has a meaning in English.
However, in Portuguese language, there's no need to translate each word, depending on the context and the type of the verb; and vice-versa.
- [position] (context)
If we analyze the movie scene again, we will realize that Dory was in the middle of the sharks, but not exactly. Thanks to the environment - the ocean - they were floating near that shipwreck; so Dory was not exactly in front of them either; just a little bit. She was not exactly down too; just a little bit.
We can't forget we are talking about a movie for younger audiences, so the actors used an specific vocabulary which helped the children to comprehend the position of the character perfectly.
Since the main topic here is the preposition, I'd appreciate if you remember that the meaning and the comprehension of each one of them, especially "up", "down", "above" and "under" are different in other contexts.
Now, let's take a look on your examples:
"I was walking down the street". = "Eu estava descendo a rua".
"I was going down the road". = "Eu estava descendo a estrada".
- [movement] (context)
Both verbs - to go and to walk - are related to the movement. So, in this case, we (can) translate all the terms precisely.
But we just have two separate clauses here. If we had a (full) sentence, the translation would be different.
Don’t get discouraged because you are doing an excellent job, my friend!
I hope I've helped.