- ik zwem = I swim (habit)
- ik zwem = I am swimming (currently happening)
As you can see you can use either the present simple or the present continuous in English. In Dutch the tense usage often is less strict than in English, the clear distinction in English you see above doesn't really exist in Dutch.
Dutch also has a continuous form (zijn + aan het + infinitive), but this tense is used WAY less than in English (only when it is very important that something is happening right now), it will be taught later in the course: I am swimming (right now) = Ik ben (nu) aan het zwemmen.
Ik zwemmen is not grammatical, it makes no sense in Dutch.
This can be translated as either "I swim" or "I am swimming". There are different tenses/conjugations (I don't know what they're called) to indicate present continuous though (aan het, 'te', etc.). 'Ik zwemmen' doesn't make sense