Several problems with this movie (spoilers)

1). The villain's plan was un-convincing. I get that she needs money, and she's an ex-con. But her plan had zero chance of success. Let's say the girls did die in there. Wouldn't the police investigate? They will see that somebody withdrew $80 from the girl's bank account. They will see that somebody checked the voice messages on the girl's phone. They will also find out that the pool had a night janitor; and there is video footage of her inside the building. From there, it's 1+1 = 2

2). The movie didn't explore the survival element enough (which is the main draw of the film). The sisters didn't adequately put their brains together; they didn't put their whole efforts into escaping. Instead of survival, the film relied on a B-level villain and the sister's non-spot arguing. And all the bickering and arguing between the sisters was so annoying. These girls are trapped in a bad spot, they need to work together. Instead, they gotta' act like idiots and argue about their lives, like they're in the family living room.

3). The conclusion made them look like idiots. So basically, all they had to do, from the get-go, was pull that crate off. They didn't do it earlier because one of the sisters acted like an idiot ("OMG! THAT WON'T WORK!"). If, like the smaller sister said, they had taken turns or pulled the crate together, then they would have been out of there a long time ago. To not make them look like idiots, they film needed to devise another way for them to escape, an idea that hadn't come up earlier.

4). Why was the pool heater still on, in the first place? Did the manager want to raise his energy bill?

5). The idea of stabbing the villain was obviously dumb. The girl wasn't going to kill the villain that way, and hurting the villain would just piss her off (which it did, leading to the chlorine). Unless you can kill the villain by stabbing her, then don't do it. Was the villain supposed to go, "Oh - they stabbed me. Well, I better let them out now"

6). The sisters realized quickly that they were dealing with a psycho woman who wanted money. Therefore, the sisters should have tried to negotiate. The sisters should have offered the ring from the start; they could explain that they'll ask the boyfriend to give the villain a reward; they could say that they'll put in a good word for the lady with the manager, the newspapers, the police, the parole officers etc. .. Also, the sisters have a legitimate claim to sue the pool manager for negligence. They could argue that they will give the villain a piece of that settlement money.

7). The way the sisters acted after they gave up the engagement ring was dumb. You're trying to survive, who cares about that ring? The boyfriend, I'm sure, will be fine with it. He'll gladly accept that you gave up the ring for your life.

8). The movie set it up as the sisters against the evil lady. But at the end, they don’t actually beat the villain. So the movie never delivers the payoff that you expect. The whole idea of turning the villain into a sympathetic character end didn’t work because we didn’t get a strong insight to the villlain’s background and motivations. She still came across as evil at the end