Your body needs water to function properly. Keep yourself hydrated while lying on the beach or playing in the sun. Drinking water may help you watch your weight when you swap out your sugary soft drinks for water. Mineral water, tap water, or bottled water are all fine, but if the bland flavor of water doesn't turn you on, add a slice of lemon, lime or cucumber.
Of course you can also get calorie-free fluids from tea and coffee, just don't load them up with cream and sugar.