Can girls use the boys showers if theirs are not working?

Do you masturbate in the shower?

Depends. Where? Ask the manager or supervisor of that place. Typically the answer would be no. Unless the facility is willing to alternate times, for example females during even hours, males during odds.

There is no law about this. I have been places where there were more women and men, and so we took over the men's room, with someone guarding the door. If this is a at a school, you have to ask the faculty/administration.