You'll need Alternate Action Clarifying Shampoo to get rid of chlorine that can build up on the hair. Follow that with Biolage Ultra Hydrating Shampoo and Conditioner to replace moisture that can be lost due to pool chemicals and sun.
Before you go out, wet your hair and work in some Ultra Hydrating Balm or regular Biolage Conditioning Balm. Then slick hair back into a tight braid if it's long enough. The conditioner will help to keep the chemicals from absorbing into your hair.
If you can, rinse your hair in plain water after getting out of the pool, so the pool water doesn't dry into your hair.

Matrix Essentials Supplemental Shampoo
Biolage Ultra Hydrating Shampoo