Yes lots of people swim in both the lake and the narrow parts. The lake also has a few sand beaches that are popular in the summer. And designated swimming areas with shallow water for kids like this: https://share.google/5bko4Xpb0tzkfEChJ

There is an app to check water quality. In general it's fine to swim in, but at high temperatures sometimes algae form. That's common in lots of these kinds of lakes in the Netherlands.