HYDRATE!!! HYDRATE!!! HYDRATE!!! Bring as much water as you can carry...and drink it! We take full camelbacks, plus 4 bottles each for a few hours out....and start out well hydrated.
(ok, you likely already know that)
We were there a few years ago early July (mtb)...it was 115 degrees. 4 am mtb rides...sweet! We drove arches and hiked around, I don't remember much in the way of water stops or any other kinds of stops, though we were carrying our own so we weren't as inclined to look. We did find a large group way back in of tourists trying to get their van unlocked with a stick. Don't know how long they'd been there, but our slimjim made quick work of it.
Hills...yes!
Found this:
http://www.utah.com/bike/trails/arches.htm
I used to have an open mind but my brains kept falling out.