I've enjoyed a couple of visits to Dutch locations which were very pleasant, and very sunny!  I was in Maastricht briefly during August a couple of years ago and managed to find my way to the Maasplassen beach which is just south of the city on the river.  It's about twenty five minutes walk upstream from the town and really lovely.  A small beach area faces into a lagoon formed by the river Maas which passes on the other side of a narrow peninsula.  There are lawns to sunbathe on and a small floating cafe for that all important Dutch (or Belgian) beer.  It was incredibly hot the day I was there and I only wish I'd had longer

My other Dutch beach experience was at Scheveningen - I think there are two beaches there but I used the one to the north side of the town.  The beaches along the west coast of the Netherlands are fantastic and this section was no exception.  There are dunes to shelter in if its windy but the whole expanse of the beach for a defined section is naturist and truly beautiful.  There are a couple of cafes in the area too.  I loved it.