i know not everyone loves this answer, but if you keep talking to her (she doesn't need to see you as long as she can hear you) it is ok for her to cry while you are in the shower. you are not leaving her to cry, you are right there reassuring her and you have to shower. there is a balance...
i hope you don't think i am being mean, i am so totally not. but if LO knows you are there and can hear you, it is OK, PERFECTLY OK, to have a shower........