There are plenty of Sea Urchins in Pulau Seribu. They are indication that the ecosystem in the sea are distorted and not conducive for turtle habitats to live.
Below is an extract from britannica.com for definition of Sea Urchins
Sea urchins have a globular body covered with movable, sometimes poisonous, spines up to 12 in. (30 cm) long. Pores along the internal skeleton accommodate slender, extensible, often sucker-tipped tube feet. Sea urchins live on the seafloor and use their tube feet or spines to move about. The mouth is on the body's underside; teeth are extruded to scrape algae and other food from rocks. Some species excavate hiding places in coral, rock, or even steel.
These Sea Urchins live on the sea floor or sticked in the corals. Always be careful when you snorkel or dive. If you did step into them, get someone to help you to get a doctor.
The Local said the first thing you can do is to knock at your feet using your hand in repetitive motion to break those spikes which has penetrated inside your feet to prevent infection
According to wikipidea, Sea urchin is a small, globular, spiny sea creatures, composing most of class Echinoidea. They are found in oceans all over the world. Their shell, or "test", is round and spiny, typically from 3 to 10 cm across. Common colors include black and dull shades of green, olive, brown, purple, and red. They move slowly, feeding mostly on algae.
The sea urchin that you would meet in Thousand Islands is the black one. Please stay away from the sting or you would get fever if touching it. Please always prepare amoniac, vinegar or...you could pee onto the area that get sea urchin strings.
If there is Tsunami.
Only God Can save you.
but still if u get enough time to try your luck.
Try to reach the BIrd Watching Tower.
1km from the beach inside the island.
The Metal is rusted but still u can try. As I said try your Luck. :-)
Check the weather report before you go to these islands if you have a plan to visit during moonson period [mid October - December]. At that moment, the waves are higher and shower rain comes down in unpredictable moments.
But okay, if you already in the boat. Just looking to the sky and find; is there any cumulus nimbus clouds formation or alto cumulus appears? Also checking is there any ripples going bigger and bigger time by time? You can predict it before the rain drops and preparing going home earlier. And don't forget to bring light rain-coat in your backpack.
be careful with this creature. in indonesian it's called bulu babi. just avoid them while you're snorkeling