We've had a few very icy looking photos and videos on here lately, but it's not always cold here. We get much warmer summers than where we used to live in the UK.
Assen is quite a way from the coast, but nevertheless we have some really lovely places to swim in the summer. The photo is of one of over 30 artificial beaches which are within Drenthe, some of which are on our cycling holiday routes.
You can swim here surrounded by woods. The water is safe as it is not too deep and it is regularly checked for quality.
If you come for a cycling holiday in the summer, you may want to bring your swimming gear too. It's quite normal to arrive at the beach by bike. In another post there is a photo of bike parking at the beach.