How to respond to aggressive behavior in a cat

I've recently moved to a new place, I share it with a married couple who are on a trip for a week, and asked if I could watch their cat. Unfortunately, the cat does not like me, and when I try to change its water it hisses and nips at me. It's declawed, so when it bats at me I try to ignore it, though I am very timid around aggressive animals. How can I get it to, at the very least, leave me alone? Thanks to anyone who can provide some insight in how to handle the situation.