Don't take a swim in the Westerschelde! This sea-arm gets it's water from the river Schelde that flows in Belgium along various industrial areas, the last beeing the nuclear plant just north of Antwerp. It's one of the most poluted water-ways in Europe.
Beeing a port Terneuzen is one of the Dutch towns which are known for the drugtrafficing. In the daytime and in crowded places you will probably notice nothing of this. Visiting the small red-light district at night is maybe not such a good idea (unless you are a sailor who has been on a ship for 3 months!). Remember that the use and ownership of small amounts of drugs is not prosecuted in The Netherlands, so don't expect a lot of policemen protecting you against any contact with drugs. Driving a car with foreign numberplates (especialy French ones) may attrackt the attention of drug-runners. This are people (mostly addicts themeselves) who try to direct you to a drugdealer. Needless to say that you should pay as little attention to them as is possible.