I think it wildly differs all around Europe.
In Spain for example, if someone says "1st floor" it can be two or three floors above the actual ground floor, if there is a "Entresuelo" or "Principal", and you start counting after those. Actual ground floor is "bajo".
On the other hand you have the "atico" (attic) which is the top level floor, unless there is a "sobre atico" ("above the attic"), so just because you live in the attic doesn't mean you live on the top floor.
Then every region can have their own convention, or even difference in neighborhoods in the same city.
Call it the Ugly European effect. https://blog.codinghorror.com/the-ugly-american-programmer/
It originally referred to attics with deeper floors:
The parts below sea-level are mostly all in the delta area. Meaning there is little risk of rivers overflowing since they are at their widest in the delta. And as for rainfall; That just means pump water harder out of the "tub" then normal. As for the threat of sea-water in these areas, that's were the Delta Werken[2] are for.
[1] https://nos.nl/artikel/2535587-noodweer-leidt-tot-overlast-w...