The people on this forum have learned that it is OK and advisable to drink water up to 20 minutes before and after the ES. The ES takes a lot of water out of your system, so you need to drink water to keep yourself hydrated.
For my first flush, I didn't know that it was OK to drink water. I woke up the next morning and felt like I had a horrible hangover. After that one, I found out about the water and have never had that hangover feeling again.