I have a love-hate relationship with "La Mer du Nord". Having grown up near the beach in Australia, the way the sea should be (waves, blue, clean), I have a little bit of hesitation when it comes to the sea in Belgium.
It is flat, kind of brown, and looks as though there is stuff floating in it, not the type of stuff you see in the Mediterranean, but sediment. My in-laws (who are French) all happily get in it and seem to enjoy themselves, while I watched from the beach. It is cold too.
One thing can be said about the beach at Oostduinkerke (not far from Nieuwpoort or equally between Dunkerque and Oostende), it seem to be always alive with activity. It is flat and expansive. People trotting (with horses of course), kite surfing, walking, playing. You can also, of course, walk your dog on the beach unlike in Australia. There are dunes to hide in and the light is beautiful. See, perhaps I do like it there after all. You can see why the French who live in the north of France likes to go there.