Please, you have every right and then some to be mad! I know I would be! Like you said, it is one thing if someone is in need of something and asks if you could help, but to just use / break your things without even asking is essentially stealing. And the fact that you are behind on your bills while they run them up makes it so much worse.
I would recommend saying something and then trying to keep them from using stuff.......maybe put the hose someplace where they can't just conveniently use it along with your water.
Of course, cool down before you go over there because the last thing you want to do is blow up and cause a huge never-ending feud seeing as you will still be living next door to them. You definitely should say something though because if they don't care whether or not to ask, they definitely won't stop just because it is wrong. I swear, it amazes me how some people act.
I can't even imagine doing something like that! Good luck and keep us posted!