While on vacation, I tend to spend quite a bit of time at the pool. Granted, I'm usually under a hat, but one of my biggest concerns is always my hair. I've said before that I spend a lot of money on my hair, and I need to protect my investment! If exposed to sun for too long, my hair becomes VERY blonde, much more blonde than my carefully (and expensively) placed highlights, and I look like a cheap bottle blonde instead.
So, I use hair sunscreen. My favorite is this FF Spray. It has a fine mist, smells like sunscreen but isn't overwhelming, and doesn't result in sticky hair. I've used this product for a couple of years now, and have managed to still look like I have expensive highlights.
Note that Frederic Fekkai also has a similar product Summer Hair Sun Bandana, which includes a conditioner. I've tried this product as well, and can only recommend it for use at the pool, since the product results in sticky hair that can't be styled.
Remember to reapply your hair sunscreen after swimming since none of these products are water resistant!
Frederic Fekkai Sun Protectant Spray