[kisito]

It seems to me that "having a shower" is used in that sentence, not as the present continuous but as a gerund. A gerund is a verb that acts as a noun. We can commonly see gerunds in words that describe hobbies and activities. For example, "I have dinner before watching TV." In this sentence, "watching TV" is seen as an activity and therefore it is more of a noun than a verb in the present continuous. Gerunds play the role of turning verbs into nouns by adding -ing.
Some teachers often confuse the use of the present continuous with gerunds. I can understand why. Of course at school we were always made to see the present continuous as auxiliary verb "to be" and verb with "-ing".
In your sentence, "having a shower" is an activity derived from a verb, therefore a gerund. Also there is no auxiliary verb "to be" in that sentence. So if you wanted to use "having a shower" as the present continuous form of a verb it would likely appear with an auxiliary verb as in this example: " I am having a shower."
The verb "to be" conjugated as "am" plus verb with -ing.