“What causes blurry vision after swimming?”

I assume you are swimming in fresh water, not sea water. Our tears normally have the same concentration of salt as sea water. Fresh water normally has little or no salt in it. When you get fresh water in your eyes, water will soak into the tissue of the cornea, the front window of the eye, because there is a higher concentration of salt in the eye tissue than there is in the fresh water. The water soaking into the eye tissue causes the tissue to swell and get a bit cloudy, which causes the blurry vision. After a period of time, your body will eliminate the fresh water, and the swelling will decrease, and your vision will return to normal. If you wear swim goggles, you will have less blurring after swimming, and you will also be less likely to catch any diseases from contaminated water.

The eye often gets some chlorine, which changes the physiology of the eye, which makes it essentially dry and blurry.

Dr. LMJ

Chlorine in pool water can irritate and dry the eyes which can lead to blurry vision.

It's due to the difference in pH and irritation caused by chlorine.