Hi Tricia!
All of my ducks drink plenty of water

They're frequently playing in and drinking water (and making a mess). I have watched them clear their nasal passages in the water by blowing bubbles, they seem to be quite good at keeping them clear. I haven't noticed any blockages externally. They are also eating well, I don't think any are overeating.
We are currently in mid-late spring and it can get up to 30-35°c during the day. The sun is also quite strong here in Australia so I wouldn't be surprised if they get a little hot in the sun with all those feathers! They have access to plenty of shade as well as an adequate amount of swimming water to cool down if they need it.
I don't think they are too stressed, they have plenty of space to roam in the backyard and they are seperated from the chickens so there's never any fighting. The only thing they have to worry about here is a few crows that come looking for the eggs. They're usually chased off by my drake and I.
She only occasionally pants and usually always when she is laying down. I'll continue to monitor her and look for symptoms of lungworm. Thank you for the helpful information!