Buy ALL the sunscreen - How much sunscreen do you think you'll need to pack for a week? Double that.
Bring Aloe Vera - You're gonna get sunburned.
Bring White Vinegar - White vinegar takes the sting out of a sunburn. Do not try to get white vinegar from the kitchen. They do not understand why you want it and you will get a shot glass of white vinegar
Use your sunscreen effectively. If you put your sunscreen on while you are getting dressed and well before you go outside, more of the sunscreen will stay on your body. If you put on the sunscreen and jump right in or after you have started sweating, you might as well pour it right into the water. Reapplying the sunscreen is not useful if you don't apply it to dry non-sweaty skin and stay that way for at least 20 min after application.