I have been to way more colorectal specialists than is normal for my age- here's what they say about water intake for an adult to help avoid constipation. Divide your weight in lbs in half: that's the total amount of liquid you should be getting in a day in ounces. 2/3 of that total should be from water. I weigh 120, so 60 oz a day of liquid, 40 oz of which should be water. That is a LOT of water- but it sure helps with bowel issues.
If you're getting all of that water and still having issues, why not try Colace (stool softener)? That's my backup.
As for Miralax, I have no experience taking it, but my 3 YO has been taking it for the last year and a half. It's a lifesaver.