The weight of a chair. (Spoiler)
Really, it seemed unlikely that the office chair would be so heavy that Ben would not have been able to lift it up to the surface of the pool with Amy tied to it. Even the heaviest things are suprisingly light in water, and we're not talking about concrete blocks here, it's a CHAIR for crying out loud! Even with her weight on it he could have easily pulled her right up. I think it's something like every ten pounds feels like one underwater. So even five hundred pounds total (which I'm sure the total weight wasn't even close to!) would have been as easy to pull upward as fifty.
I know, suspension of disbelief for plot purposes, but really, for anyone who's spent ample time in a pool as I have, including exercises that include heavy weights, that's just pushing it a little too far for unbelievability. When I saw him try to pull her up and seeing that the chair was apparently far too heavy and weighted down, all I could think was "oh give me a break!"