Swim can be a verb and also a noun. Why in the sentence below, has "swim" "ing", because we can use swim as a noun without "ing"?
My favorite sports are football and swimming.
The noun "swim" that you are referring to can only occur with a determiner like "a" or "my" ("I’m going for a/my swim", but not *"I’m going for swim").
But the noun "swimming" that is formed by conversion form the verb form "swimming" behaves differently. This noun may require a determiner, as in "The swimming here is very cheap", or it may not, as in your example.
Sometimes we cannot be certain whether "swimming" is functioning as a noun or a verb.
For example, "I like swimming" is strictly speaking ambiguous, but the verb is the more salient interpretation ("I like to swim"). However, noun interpretation can be forced by adjectival premodification, as in "occasional swimming".