I think if using "Are you swimming today", the translation would be "Schwimmst du heute?" Here schiwimmst is absolutely a verb. But in the correct anwser, I guess gehst(gehen) schwimmen is a set phrase. btw, I'm learning German too, all the above is my own understanding. ;)
I put "Are you going swimming today?," & It wasn't accepted. My sense is that this is the most common phrasing in English and that it is equivalent to "Are you going to swim today?" In other words, for this kind of construction, the present participle and infinitive are, for all intents and purposes, the same. Other examines: Are you going running, dancing, jogging, sailing, etc.
See here: http://canoo.net/services/OnlineGrammar/Wort/Verb/Finit-Infinit/Infinitiv.html?MenuId=Word21200#Anchor-Reiner-47857 Apart from modal verbs, there are some other verb classes that go without the 'zu'.
Oh, ok. Yeah, DL accepts a lot of sentences that while grammatical aren't used much. And also, you don't know what someone's native language is. My native language is english, so I love getting advice on what is natural in german because once you know the words and the grammar little things like that really make your language that much better :)