You can drink water till 20 min before the 1 ES and again after 20 min. Most people don't drink between the 2nd ES and OO.
You should read the posts on the liverflushforum and also the FAQ at the top of the page, it has a lot of information.
This is more a forum for people who want or did surgery. The other one is for people who decided they want to solve their problems without surgery.