Yes chickens can swim very well - but they don't like the water and will get out as soon as they can. I have seen a chicken try to fly over my lake, not make it and it landed in the middle, It then sort of flapped, swam to the other side and got out.
I have a small fish pond in my garden, and my Japanese Bantams love to paddle about in the shallow end. Sometimes one will walk out too far in the lilly pads, and fall in - they never learn. But then just swim / flap at bit and they are out in seconds.
If the creek does not have steep sides, and is not fast flowing, you don't have anything to worry about for adult chickens.
However, young chicks can easily drown as they are weak swimmers and get cold quickly.