What is observed here is the thermal radiation, which is modified in its interaction with the dectector; the water is merely implied. So the observer effect is respected - the thermal radiation itself was modified in the act of observation.

But there is a key difference between the observer effect and HUP. In particular, we can take account of the observer effect to calculate what we wanted to know. For example, in this case we effectively "destroy" the thermal radiation by absorbing it, and conclude what the thermal radiation was before we destroyed it. Whereas HUP says that there are things we inherently cannot know.

If the thermal radiation detector is anywhere near the water, you will have to be very quick before the detector sends some thermal radiation to the water and modifies its temperature.