There are many beaches dotting the coastline of kristiansand. I suppose the 2 biggest and most popular are Hamresanden and the beach in the city.
Hamresanden is really useful if you want to swim in the sea as well as a river - as it has both. There is a large camping ground at Hamresanden and you can drive into this area to get to the river side of the beach or simply park near the water on the sea side of the beach. You can get food there or there is a supermarket across the road. The city beach is also nice, but doesn't really feel like a beach.
The picture above is of Hamresanden beach.
Here is a video of kids swimming at Drangsholt. It is located not far from the airport. There are many places to find a little private beach or swimming areas in Kristiansand. It is nice to take a swim in a fresh water lake on a hot summers day!
The front beach is to the left and around the point is the river side beach.