Learn Farsi: در تابستان شنا می‌کنم.

در (in) is the standard preposition for seasons, months, and years. However, in casual spoken Farsi, it is very common to drop it entirely and just say تابستان شنا می‌کنم to mean the exact same thing.

Where is the pronoun من (I) in this sentence?

Farsi is a pro-drop language, meaning the subject pronoun can be omitted because the verb ending itself tells you who is doing the action. The -م at the end of می‌کنم (I do) makes it clear that the subject is I. Including من would simply add extra emphasis.

Since شنا می‌کنم can mean both I swim and I am swimming, how do I specify if I am swimming right now?

While context usually makes it clear, if you want to strongly emphasize that the action is happening exactly at this moment, you would use the auxiliary verb داشتن (to have) and say دارم شنا می‌کنم. In our example, the phrase در تابستان (in summer) clearly shows the regular present tense is being used for a habit.