First and foremost is sunscreen. I always pack a body sunscreen oil from Clarins and face sunscreen from Shiseido or Murad. I will apply these multiple times throughout the day because I follow Dr. Lancer's mantra, "There is no such thing as a healthy tan". I will also take a facial mist, either from Caudalie, Avene, or Shiseido to use when my face gets hot from the sun. When you leave the beach or pool, you can apply a good post-sun moisturizer like the After Sun Greek Yoghurt Cooling Gel from Korres. If you are worried about sun-damaged hair, you can use the Clarins sunscreen oil for that as well.
I love my beach towel from Zara Home. It has a really subdued, elegant look which makes it stand out in the right way.
I've packed a pair of inexpensive mirrored sunglasses so that they look cute but if they get lost in the waves, I'm not devastated. I also packed a hat in case my forehead gets too hot, a pair of leather sandals, and a gauzy cover-up.
I'm currently reading the fourth book in the Chief Inspector Gamache series and, of course, took it with me to the beach. I mentioned a few beach reads before but some other suggestions are Invincible Summer by Alice Adams, The Sixteenth of June by Maya Lang, and American Wife by Curtis Sittenfeld. For quick reads, I like to pack a copy of W Magazine or Elle.
My go-to snack for the beach or pool is a bag of frozen grapes. I also like to make a pitcher of water infused with lemon, lime, berries, mint, ginger and cayenne pepper---a sort of detox lemonade I picked up from the Tone It Up girls--and pour it into individual bottles.