I'm a native speaker of American English, and I find "They swam against the stream" to be unfamiliar. If it is meant metaphorically, the saying is, "They swam upstream". If meant literally, one would say, "They swam against the current." The latter could also be used metaphorically, but it is less common.
I am native AmEng speaker also (NE, rural Florida), and my initial thought on this was "they swam against the stream" in all its metaphorical glory. I answered, though, with "they swam against the current" as that is more literal, and still maintains the idiomatic meaning--I just find it a little less colloquial.
My natural inclination is to use "go against the tide". I think the preferred wording is highly region-specific.
"They swam against the stream" is fine. If you wrote something else, probably Duo's "Correct sentence" just happened to use "you" instead of "they." Doesn't mean "they" is wrong; Duo just shows you some (possibly somewhat random) correct answer.
What was the entire answer you wrote?
More accurately, "geschwommen" is "swum." It's a past participle, so we can't just use it as a verb. (It would be like saying "I eaten some cookies yesterday.") "Sie geschwommen gegen den Strom" is wrong because it has no verb. But you could say "Sie sind gegen den Strom geschwommen."
"Schwammen" is the preterite / simple past form, which is used mostly in writing. "Sind ... geschwommen" is the present perfect / compound past form, which is used mostly in speech. For more info on this, see here.
As a native speaker of AmEnglish, I find that the words "stream" & "river" are offten synonomus in their meaning, the only difference being that "river" is perhaps used when pointing out exceptionally large "streams". Is this the same in german? I.E. "are "Strom" & "Fluss" compleatly interchangeble? Also, does german have a word for what I would call a "creek" (a very small river typically no more than 10 or so feet across)?
River and stream both refer to flowing water, but I don't know anyone who considers them synonymous. Rivers are big. Streams are small.
But how big? How small? Well, that's awfully subjective, isn't it. I grew up on the St. Johns River in Florida. She's about two to three miles wide. My friends and I laughed when we saw the Broad "River" in South Carolina for the first time: it was about 180 feet wide. I'd have never considered something like that to be a river. In fact, we were familiar with Rice Creek, and that is 300 feet wide, 50% bigger than the Broad River.
I've learned since then that what we thought was a normal-sized river is in fact rather large as rivers go.
I might have said something that suggests I know something about German. Sorry.
In English 'You swam against the stream.' leaves open the question, "Stream of what?" An idiomatic expression would be 'against the current.' You wouldn't 'swim against the creek.' You could swim up stream, i.e. up the current of the water, probably in a river.
"Against the stream" has fallen into disuse in comparison to "against the current" and "against the tide". All three are idioms and refer to the difficulty of swimming against a flow of water. (As an aside, "against the tide" is technically incorrect because tides rise and fall, so swimming against it means one is either swimming downwards or upwards, which is not what is intended by the phrase.)