Yeah, any visitors to the dirty green bathwater are very welcome & become "ours" by default! But the plumpness you see might be from the splashing around. It's so funny to watch but tricky to capture in photos.
We are convinced they must be "our" pair, two left over from the nest in the spring. They have learnt the good places to hang out, the same ones as when they were being fed as babies. It's heart-warming to see that they still go round together, politely taking it in turns for the jaccussi!
Presume they will each need to go their own way soon:-(
This might well be one of them. If not, they would have looked exactly like this as a baby: