UV radiation affects your hair, just like it does your skin. So you have to protect your hair, and scalp from the sun.
Use a really gentle, moisturising shampoo. Baby products work well for this. Your hair is already stressed out from sun and salt water, so be nice!
Baby shampoo
Then use a really rich moisturiser after. Don't hesitate to go for "dry hair" products like Shea Moisture. Your hair needs a little more at the beach!
Rich conditioner
Lastly, put some of your favorite hair oil on the lengths and tips of your damp hair. This can help lock in the moisture you just provided.
Hair oil