Could've Been really Good
As I was watching I noticed the nod to Burnt Offerings and the pool scene. I was intrigued. It also, at that point, reminded me of the Twilight Episode The Bewitchin' Pool which I was hoping would be the direction it took.
If they had spent more time on the background of the Entity it could've helped. I know they visited the mother of Rebecca and we sort of got the motivation. But I would've liked more. However, I actually liked it more than other people. But it definitely could've been better.