Netherlands. Province: Zeeland. River mouth: Oosterschelde. Is closed since the 1970's i think but wasn't back in WWII.
Northern Netherlands: The "Afsluitdijk" was build in the 1930's since then it has been impossible to enter het "Ijselmeer" except through locks.
Northern Netherlands: Lauwersmeer wasn't closed off back then.
Back south a bit the port of Rotterdam: The "Maas vlakte" wasn't infront of our coast. But only since 1973.