Sabina In summer. But think it's end of August now. In September outdoor pools and outdoor temperatures as such will be much more moderate, one could argue "not heat".
And as you mentioned, Poland has sea beaches in rather "nordic" regions.
(And I refuse to acknowledge the reason why we tend to have a traffic jam in Vienna on weekends on the city Autobahn, that is totally random, has has nothing to do with half of Southern Europe going North and half of Northern Europe driving South. Absolutely random happenstance that all these non-Austrian licence plates are blocking the traffic.)