Has there ever been any episode in Star Trek where they showed a bathroom?

Yes, there have been several episodes of Star Trek that show a bathroom. Here are a few examples:

* In the Star Trek: The Original Series episode "The Cage", Dr. McCoy is seen using a bathroom with a sink, toilet, and shower.

* In the Star Trek: The Next Generation episode "The Naked Now", Commander Riker is seen using a bathroom with a sink and toilet.

* In the Star Trek: Deep Space Nine episode "In the Cards", Quark is seen using a bathroom with a sink and toilet.

* In the Star Trek: Voyager episode "Threshold", Captain Janeway is seen using a bathroom with a sink, toilet, and shower.

* In the Star Trek: Enterprise episode "The Andorian Incident", Commander Tucker is seen using a bathroom with a sink and toilet.