I asked a native English speaker that question. He told me that both "the shower is out of service" and "the shower is out of order" are correct sentences. However, they would only be used if the shower was public"
And "out of service" is offered as the first choice by the "help", so it SHOULD BE ACCPTED!
I'm not a native speaker, but as a Slavette I can say that it is probably right
i like to purposely confuse душ and душа in the nominative. it can be pretty funny sometimes