Translation:Let's go captain, we take a shower.
No: it's a bit more complicated. In English certainly we would say 'I am going to have a shower,' or 'Are you going to have a shower?', or 'I have had a shower,' all meaning that the subjects have showered or are going to have a shower. But that usage does not apply when the pronoun 'we' is used. Again you would say 'We have had showers,' or 'We are going to have showers', but the expression 'We have a shower,' can only be possessive.
I don't think so, because we don't refer to christenings as "showers." Only certain celebrations count as "showers" - baby showers, as you say, and bridal showers. Why, I have no idea...(maybe because the parents or bride to be are "showered" with presents, whereas you don't shower a boat with presents when you christen it?)