Henk,
Several Spitfires have come down in the Westerschelde, so without further info, it's hard to tell which one you're looking for.
You can find losses in the Netherlands here:
http://www.nimh.nl/nl/collecties/Documentatie/
Just click on any of the years and a full list will open up with Westerschelde as crash location in several cases.
Regards,
Leendert