I know I've offered this opinion before but I can't resist doing so again because it is something I feel quite strongly about.
Taking Letoon as an example.
We stayed there many times, always outwith school holidays, but also always when it was fairly full.
We seldom found the pool overcrowded and the number of sun loungers, whilst limited, to be adequate.
Now if we went out for the morning and came back for an afternoon by the pool to find all the loungers taken by residents then that would have been tough luck.
But on the other hand if we came back and found that there were no loungers available because, for the sake of argument 4 or 6 were being used by non residents, then I confess I would have been miffed.
The question I would ask the non resident would be “if you wanted to spend time by a pool during your holiday, then why didn’t you book accommodation with a pool”.