It is vital to apply enough sunscreen. In order to receive the protection offered by an SPF15 sunscreen, you would need to put on a 120 ml bottle every day you spent at the beach. Most people do not use nearly that amount, which will reduce the SPF considerably. Moreover, the effectiveness is likely to diminish further when you perspire or wipe your skin with a towel. You should reapply sun cream at least every hour and after swimming, even if you are using a waterproof brand.