Drink plenty of water.
This is mostly common sense. You should always drink water when you are in the sun and it is hot and humid so your body doesn’t get dehydrated.
Drink coconut water:
Of course you cannot always get a coconut at the beach unless you are in some exotic place, but local grocery stores also carry coconut water. It is not quite as effective as plain winter, but if you need something with a little flavor, this might be a good option.
Eat Fruit:
Many fruits are rich with water. Watermelon is the fruit with the larges amount. If you bring fruit to the beach, the kids will love it and they will be staying hydrated.
Here is a great article about foods that are good for hydration.
