I've heard about slathering conditioner on your hair before swimming but I haven't heard of washing it. I wouldn't wash it if it were me just because the cleaner the hair is, the more absorbent it is and I wouldn't want my hair sucking up all of the chemicals from the water (unless you're at the beach in which case the water could be beneficial to your hair).

But I would just use a pre-shampoo oil such as coconut or olive oil. When you're in the water you have protection and when you shampoo afterwards your hair is protected further from the shampoo drying it out. Then I would do a heated deep treatment after for a few minutes just to be sure.