Learn Portuguese (Portugal): Ela nada na piscina.

Yes, it does! But here, nada is the "she" form of the verb nadar (to swim). You can tell it is acting as a verb here because it comes right after the subject Ela (She).

Why is "in the" translated as na?

na is a contraction, which is a mandatory mash-up of the preposition em (in/on/at) and the feminine word for "the", which is a. Because piscina (pool) is a feminine word, we must use na instead of no.

Since we are learning about the preposition de, why don't we use it here?

The preposition de generally means "of" or "from", or connects to specific verbs like gostar (to like). Because we are talking about an action happening physically inside a place (the pool), we need to use the preposition em instead, which becomes na when combined with a piscina.