My wife and I put water-based polyurethane on the floors of our rental unit yesterday, and most of the house dried fine, but one area dried looking milky/cloudy. I have read that this is due to the clear coat trapping moisture inside and drying too quickly on the surface. We have almost no time to resolve the situation, as the tenants move in on Friday. Is there any way at all to even mitigate the effects? Buffing, polishing, light paint thinner?

Did you make sure the floor was absolutely clean before you applied the finish?
A warm room with a fan to circulate the air is the best way to help the floor cure, if that is your problem.