You have probably seen that the DHT-22 sensor promises an accuracy of +-0.5°C for temperature and +-2% for humidity. So, if are building almost a sort of life-support-system, you may want to think about redundancy, e.g. by joining 2-3 sensors per unit and taking their average value or calculating their relative deviation to avoid surprises and notify you about sensors going awry.
I was actually going through the same thought process as you did, @Birdy: solar-powered floating sensors measuring all sorts of things in my pool.
I also looked at pH sensors to test water quality, but the recommended recalibration intervals ranged somewhere between a couple of weeks and before each use. Sounded a bit too complicated.
The pool sensor use-case I'm currently focusing on is measuring the level of remaining liquid chlorine in a barrel behind my pool connected to an automated chlorinator. I looked at various different options and settled on a waterproof ultrasonic sensor to be built into the top of the barrel. I'm planning to combine this with a waterproof temperature sensor hanging on a long leash in the pool.
The pool season has just started and I hope to get that all built before it ends. 
Have you followed the recommendation in the documentation to send an initial value to the controller? The symptom you're describing sounds as if that may not be the case.