"swim good" is poor English. Good is an adjective (a good swimmer) , well is an adverb (swims well)
i can think of only one case where you can get away with using good in place of well: "I'm good / I'm doing good." and even then some people will give you a hard time for it.
Replacing the adverb "well" with "good" is often times used, however it still is wrong. However a lot of people would considerate it colloquial informal speech.
''good'' English grammar absolutely requires the adverb ''well'' in this translation.
Hard for me to hear the "nadan". Wouldn't it be great to have an alternating male and female speaker?
This sentence is describing the action, while your sentence is describing the students. Does that make sense? So it would be a slightly different meaning. The translation of yours would be something like, "Los estudiantes son nadadores buenos."