Even though you are surrounded by water at the beach, you can become easily dehydrated, especially if it is sunny. Make sure you pack enough water to last you for your trip, and try to drink it somewhat frequently. The last thing you want to happen is that you become dehydrated while you are swimming in the ocean.  Bring along a cooler to store your drinks in so that you can keep them cool. Also, try to avoid drinking too much alcohol without drinking any water. If you are going to drink alcohol at the beach, make sure you drink enough water and eat enough food to counteract the effects of the alcohol. Swimming in the ocean while being buzzed or drunk can be very dangerous. Before you bring drinks, look at the local beach's rules. Some beaches may not have any regulations when it comes to what you can bring but many prohibit their guests from bringing glass bottles. If you are spending hours at the beach without sunscreen, you will most likely get burned. Make sure you apply sunscreen before you go to the beach, but pack some too so you can apply it regularly throughout the day. Water and sand are major reflectors, so they reflect the sun's rays, increasing your chance of burning. Apply more sunscreen than you would think, just to be safe.  Try to use a waterproof sunscreen so that it doesn't wash off in the water. Aim for a sunscreen with an SPF of 30 or higher with broad-spectrum protection that protects skin from UVA and UVB rays. Apply your sunscreen at least thirty minutes before going out in the sun, and then every two to three hours that you are outside. Don't forget to apply sunscreen to all areas exposed to the sun. Wear lip balm with an SPF, and try to apply sunscreen to your ears and scalp if possible.  Even if you are wearing a t-shirt in the water, apply sunscreen to your body. If you are going to be at the beach a while, it may be a good idea to pack a lunch or bring snacks to munch on throughout the day. Some beaches, the more crowded ones, will have concession stands or vendors selling foods,...

TL;DR in Indonesian:
Bawalah air putih. Bawa dan kenakan tabir surya secara rutin. Buatlah makan siang atau bawalah camilan. Bawalah baju ganti dan handuk. Bawalah kursi pantai atau kursi taman dan beberapa handuk pantai. Bawalah barang-barang untuk hiburan. Sediakan perlengkapan P3K.