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.