I summer, it seems to us that Østfold is the place in the world with the best weather and the clearest, cleanest air.  This is probably not true, but on a warm summers day, there is a good chance you would like to go for a swim.

Indoor pools
Swimming indoors is available in Sarpsborghallen, run by the county.

At the Quality Hotel at Grålum, you will find a indoors waterpark (and plenty of other activities).  Pools, water slides, jacuzzi, rapids..

Swimming outdoors
Fact: There is more room and more water outside than inside.  And in Østfold you'll propbably find what you're looking for.  Sarpsbork Tourism will tell you where the best places are.   Do you prefer naturism, the place is Sjøhaug at Jæløy near Moss.  Høysand in Skjeberg is also nice, and of course Feriehjemmet - an old vacation spot for employees from Borregaard, now open for the public.  Here you'll find camping, kiosks, piers and a café.

You also got several nice beaches on the islands and the archipelago around Fredrikstad, Halden, Moss and the surroundings.

A popular freshwater alternative in the immediate area, is Tunevannet, close to Sarpsborg center by the beautiful Tune Kirke.  There is an open kiosk in the summer, and is a very family friendly place to go.

A little further north, you'll find Børtevann, where there is café open most of the seasin. A grill is also available here.