Imagine if your baby was a 3 or 4 year old, with a vocabulary. The equivalent of the crying would be them saying "ah but Muuuum!!! I don't WANT you to have a shower, I want you to play trains with me!" What you'd then say, quite reasonably, is "yes, I will play trains with you but AFTER I've had my shower, I will only be 10 mins, you can sit there and chat to me if you want or look at your book until I've finished." You would know they are fine to wait and wait they can, without any ill effect.
If, on the other hand, your 3 or 4 year old was crying and saying "Mum! Why are you going in the shower? You've ignored me for over an hour and you didn't give me any breakfast and it's nearly lunchtime and I'm so so hungry. And I've just had an accident too so need help with cleaning myself up" and you still ignored their needs and went in the shower anyway, THEN they would have a point, wouldn't they?!
The point is, though, the fact that your baby has no vocabulary yet to express their displeasure at you not pandering to their every whim is not your fault. They are too young to understand that there is nothing wrong whatsoever with you having a shower, they have all their needs met and they can safely wait without any ill effect at all. They WILL come to realise all this, but there is no need to be hard on yourself and torture yourself with the misguided thought that you are somehow harming them emotionally. You aren't. You simply need a shower. Their world isn't going to end by you chatting to them for a few minutes telling them that eg "I'm putting the conditioner on now, that makes my hair all soft, you like to twiddle it, remember? It smells lovely! Then I grab the sponge and use this yummy shower gel that gets me all clean. " etc etc Totally inane crap lol. But they hear that mummy is fine and happy and bright and breezy and not all stressed-sounding going "oh you sound so upset, I won't be long, Mummy is hurrying as quickly as she can!"
It will all be fine. Have your showers, and have them totally guilt-free.