Es presente, "una vez por semana" es lo que lo delata... Indica que es algo que hago en mi vida cotidiana, una vez, concretamente, a la semana. Además "voy" es presente del verbo ir. La idea de futuro se expresarla tal k: "iré a nadar una vez por semana" y "nadar una vez por semana",,,,una idea de 'presente continuo español' podría ser "voy a ir a nadar una vez por semana"
Yes, that is present indicating a habit. Unfortunately. it is possible because of the "once a week". Perhaps they were originally trying to catch us to see if we knew the difference between the two forms, but now they also accept "I am going once a week." which is the intention of planning to do that.
Yes, this lesson is on the future, just as (for me) the two previous ones (No. 3 and No. 4 in the "Future Tense" module were. Yet not one sentence in any of these lessons has used any "future tense" examples. They've all been rehashed examples of the "future phrasal" (ir + infinitive) instead. Very annoying.
Para mí es presente (arriba doy mi argumento), y como tú dices para mi lo correcto en inglés sería "I go swimming once a week" ya que "una vez por semana" es lo que, en mi opinión, hace que sea una acción de la vida diaria y por tanto sea presente inglés en lugar de presente continuo /progresivo... (si fuese "voy a nadar" sin nada que especifique la acción 'temporal' si admitiría el presente y el presente continuo inglés, véase: "I'm going to swim" y "i go swimming" ,,y si fuese "voy a nadar con mi amigo" también admitiría las dos...
This happens a lot in other languages, because in English we use the progressive tense more often than they do in other languages. "once a week" allows the present habitual form, but "voy" can be either "go" or "am going". It is unfortunate that they put that little phrase into a lesson that is often captured into the future section of lessons.
Both are accepted as correct. "once a week" definitely allows the present that we use for repetitive actions, but we would often say "I swim once a week." Still, if you wanted to explain that you were not home every week, but swimming at the pool elsewhere, you could say "I go swimming once a week." I would assume that that the pool was elsewhere than your home. You could say "I am going swimming." as an intention of near future and that could be anywhere, including in a pool in your backyard. I understand your confusion if you had encountered this sentence in the future section. Both are accepted as correct. Think of "I am swimming." which can also mean both present or future in English. In English we can say, "I am swimming now." and "I am swimming tomorrow." for both present -currently happening and near future, even though we have a future continuous "I will be swimming." which is just not used very often for the near future.
The present continuous can express a future: http://www.grammaring.com/present-continuous-for-fixed-future-arrangements
I think it can have two meaning here, with no context (I hope someone will correct me).
1/ The person plan to go to the swimming pool (or the sea, etc) once a week, but it's not the case right now. Planning. Future.
2/ The person has already the habit to go once a week to the swimming pool. Present.
Yes, if you encountered it in the future section, you would feel like it shouldn't be there, but the algorithm may find sentences with "voy" and add them to this lesson. It is unfortunate that it picked a sentence with "once a week" in it. You will often find a sentence that does not seem to belong to a section as if Duolingo were testing us to see if we know the difference between tenses.
The Spanish "voy" can mean either "go" or "am going", so it is a correct translation for this sentence, even though it is not a future translation.