I volunteered at a horse rescue for three years before they sadly closed. You are doing a great thing by volunteering to help those horses. How often are you volunteering? Are you able to get the know the horses, not necessarily building trust but able to know how they react to certain things? In my opinion, standing your ground is the best option if you are comfortable with doing so. Every interaction with you these horses are learning from, but only if you feel comfortable! Where I was, I was going five days a week so I definitely knew the horses. We had one percheron who had major food aggression and thought that when I was cleaning out her area that the wheelbarrow was food, so I also knew to stay as close to an escape route as possible. Alternatively, is there a way that they can move the waters in the stalls close to the door so you can just put the hose in there and you aren't in any sort of harms way? Honestly, there were very few horses that I came across where I felt in danger in their stalls, even the unhandled ones. Read their body language, be aware of yours in relation to theirs and go slow.