During the rainy season is danger to swim in the river because the river head may suddenly have big current flow strong . Beware of currents and rip tides. Sumatra doesn't have the same dangerous currents as along the south coast of Java though. Be careful with open wounds especially from coral reefs. They heal slow and get easily infected due to the tropical climate and the mobile lifestyle a tourist keeps. Clean and disinfect. Mercurochrom ( Obat Merah ) is recommended in your medicine kit.
In the oceans are many sharks, but the most dangerous ones , for example the Great White shark , prefers a bit cooler water . It is extremely rare in Indonesian waters . Common reef sharks are not considered dangerous. Sharks that feel threatened can attack. Always stay calm and leave the area . The barracuda is less dangerous than the shark , but can be attracted by flashy jewelry and speared fish . The Indonesian barracudas are not aggressive as their relatives in the Atlantic though . Sea snakes are the most poisonous of all snakes , but not aggressive . Their teeth are very short and placed far in and can only bite small objects . The saltwater crocodile is an aggressive creature . They live normally in murky estuaries and not at coral reefs and moving waters . Crocodiles are extinct in most rivers or afraid of humans, however common in the Alas River near Singkil. Crocodiles are probably the most dangerous animals in Indonesia, with the exception of the mosquito, when looking at statistics. When you are biten by the mosquito ,drink much water , some fresh juices to relief fever. Mosquito such dengue or malaria only live in flat area but on hilly places just forest mosquito.
Stingrays and scorpion fish should be taken seriously. The stingrays often hide in sandy areas near the shore. They normally flee when a human is coming, but sting if they are suddenly stepped upon. Using flip-flops doesn’t protect. The sting is rarely fatal for an adult, but causes much pain and fever. Get the victim to the beach and remove any spine still left. Wash and slightly bleed the punctured area. Immerse the wound in hot water, up to 50 degrees or put on hot compresses. See a doctor. If you are too far from a doctor, consult a local healer. They are often experienced with these kinds of problems. Scorpion fish and Stonefish are poisonous and camouflage themselves. The stonefish can be deadly. Sea Urchins, especially the black ones, are better avoided. Be careful when you walk over the reef. They can be painful to step on. The spines can be dissolved by urine. Jellyfish are not a problem in Indonesia.
Avoid activity in the river in rainy season when every night there is big rain because suddenly can be there is big wave from up side of the river that can sweep you away strongly and can be fatal. Rainy season start from September till first week of January.
Diving and snorkelling are best on Pulau Weh.
Surfing is best on Nias island , Lok Nga beach ( Aceh ), Cubadak island , Mentawai Island and Telos island.
Going down a river on a tube, so called tubing, can be a lot of fun but can also be dangerous, especially in the rainy season. The American Canoe Association has issued safety recommendations. In short: Never go tubing alone. Use a guide with swift water license if available and if you prefer. Don’t tube when the river is flooded. Look out for strainers in the water, for example fallen trees, bridge pilings, etc. Avoid dams, ledgers, etc. Don’t use drugs or alcohol before tubing. Consider using a lifejacket and helmet.
There several organizers of rafting and there are several rivers suitable for it. The most well-known and convenient is Bukit Lawang, but Ketambe and Asahan are also interesting alternatives. Most rafting is done on Sungai Wampu.